Enable generating multiple combined projects.
Enable generating multiple combined projects.

Summary:
Since combined projects are now output to a unique directory in
buck-out we can generate multiple ones in the same command
invocation.

Test Plan:
buck project --combined-project target1 target2
Verify both combined projects are generated correctly.

LegNeato/buck,raviagarwal7/buck,MarkRunWu/buck,LegNeato/buck,janicduplessis/buck,siddhartharay007/buck,1yvT0s/buck,hgl888/buck,Learn-Android-app/buck,siddhartharay007/buck,MarkRunWu/buck,zhuxiaohao/buck,k21/buck,vschs007/buck,Dominator008/buck,tgummerer/buck,shs96c/buck,davido/buck,clonetwin26/buck,LegNeato/buck,siddhartharay007/buck,mikekap/buck,nguyentruongtho/buck,shybovycha/buck,luiseduardohdbackup/buck,neonichu/buck,artiya4u/buck,zhan-xiong/buck,liuyang-li/buck,pwz3n0/buck,rmaz/buck,mnuessler/buck,illicitonion/buck,brettwooldridge/buck,rhencke/buck,rhencke/buck,vine/buck,vschs007/buck,sdwilsh/buck,dpursehouse/buck,dsyang/buck,ilya-klyuchnikov/buck,grumpyjames/buck,ilya-klyuchnikov/buck,liuyang-li/buck,zhuxiaohao/buck,raviagarwal7/buck,darkforestzero/buck,neonichu/buck,mogers/buck,shybovycha/buck,robbertvanginkel/buck,JoelMarcey/buck,vschs007/buck,Learn-Android-app/buck,darkforestzero/buck,pwz3n0/buck,robbertvanginkel/buck,hgl888/buck,tgummerer/buck,robbertvanginkel/buck,grumpyjames/buck,Heart2009/buck,artiya4u/buck,mogers/buck,janicduplessis/buck,Addepar/buck,siddhartharay007/buck,darkforestzero/buck,shybovycha/buck,shs96c/buck,1yvT0s/buck,sdwilsh/buck,illicitonion/buck,dsyang/buck,zhan-xiong/buck,pwz3n0/buck,nguyentruongtho/buck,pwz3n0/buck,janicduplessis/buck,Addepar/buck,SeleniumHQ/buck,shs96c/buck,marcinkwiatkowski/buck,Heart2009/buck,Learn-Android-app/buck,vschs007/buck,SeleniumHQ/buck,SeleniumHQ/buck,zhan-xiong/buck,nguyentruongtho/buck,janicduplessis/buck,vschs007/buck,liuyang-li/buck,darkforestzero/buck,ilya-klyuchnikov/buck,sdwilsh/buck,Distrotech/buck,grumpyjames/buck,sdwilsh/buck,kageiit/buck,Dominator008/buck,raviagarwal7/buck,mikekap/buck,rmaz/buck,Dominator008/buck,mnuessler/buck,dsyang/buck,rmaz/buck,illicitonion/buck,marcinkwiatkowski/buck,romanoid/buck,illicitonion/buck,LegNeato/buck,vschs007/buck,bocon13/buck,rhencke/buck,illicitonion/buck,shybovycha/buck,shs96c/buck,zhan-xiong/buck,Dominator008/buck,davido/buck,brettwooldridge/buck,justinmuller/buck,Addepar/buck,OkBuilds/buck,romanoid/buck,raviagarwal7/buck,siddhartharay007/buck,Distrotech/buck,shs96c/buck,mogers/buck,kageiit/buck,lukw00/buck,janicduplessis/buck,ilya-klyuchnikov/buck,Dominator008/buck,k21/buck,janicduplessis/buck,dpursehouse/buck,mnuessler/buck,Learn-Android-app/buck,artiya4u/buck,rowillia/buck,Dominator008/buck,Distrotech/buck,brettwooldridge/buck,brettwooldridge/buck,artiya4u/buck,rowillia/buck,zhuxiaohao/buck,romanoid/buck,neonichu/buck,dushmis/buck,davido/buck,tgummerer/buck,pwz3n0/buck,davido/buck,grumpyjames/buck,SeleniumHQ/buck,Addepar/buck,luiseduardohdbackup/buck,luiseduardohdbackup/buck,k21/buck,liuyang-li/buck,hgl888/buck,justinmuller/buck,grumpyjames/buck,daedric/buck,Learn-Android-app/buck,LegNeato/buck,Dominator008/buck,siddhartharay007/buck,clonetwin26/buck,darkforestzero/buck,OkBuilds/buck,mogers/buck,davido/buck,mnuessler/buck,Heart2009/buck,hgl888/buck,ilya-klyuchnikov/buck,grumpyjames/buck,rhencke/buck,darkforestzero/buck,neonichu/buck,shybovycha/buck,MarkRunWu/buck,daedric/buck,facebook/buck,shybovycha/buck,1yvT0s/buck,zpao/buck,dushmis/buck,kageiit/buck,OkBuilds/buck,shybovycha/buck,ilya-klyuchnikov/buck,dsyang/buck,daedric/buck,rowillia/buck,zhan-xiong/buck,neonichu/buck,stuhood/buck,zpao/buck,1yvT0s/buck,darkforestzero/buck,justinmuller/buck,marcinkwiatkowski/buck,nguyentruongtho/buck,davido/buck,mikekap/buck,clonetwin26/buck,sdwilsh/buck,JoelMarcey/buck,LegNeato/buck,justinmuller/buck,bocon13/buck,mogers/buck,rowillia/buck,k21/buck,robbertvanginkel/buck,hgl888/buck,shybovycha/buck,stuhood/buck,mikekap/buck,mikekap/buck,raviagarwal7/buck,clonetwin26/buck,JoelMarcey/buck,Learn-Android-app/buck,bocon13/buck,SeleniumHQ/buck,siddhartharay007/buck,tgummerer/buck,facebook/buck,hgl888/buck,liuyang-li/buck,artiya4u/buck,Addepar/buck,JoelMarcey/buck,dpursehouse/buck,lukw00/buck,mogers/buck,Heart2009/buck,illicitonion/buck,pwz3n0/buck,bocon13/buck,dushmis/buck,lukw00/buck,k21/buck,bocon13/buck,liuyang-li/buck,SeleniumHQ/buck,luiseduardohdbackup/buck,vschs007/buck,pwz3n0/buck,facebook/buck,darkforestzero/buck,clonetwin26/buck,raviagarwal7/buck,rmaz/buck,hgl888/buck,zhuxiaohao/buck,ilya-klyuchnikov/buck,davido/buck,zpao/buck,OkBuilds/buck,mikekap/buck,dsyang/buck,1yvT0s/buck,OkBuilds/buck,bocon13/buck,pwz3n0/buck,k21/buck,rmaz/buck,artiya4u/buck,Distrotech/buck,dpursehouse/buck,tgummerer/buck,lukw00/buck,Addepar/buck,dushmis/buck,clonetwin26/buck,LegNeato/buck,rmaz/buck,vschs007/buck,SeleniumHQ/buck,1yvT0s/buck,k21/buck,hgl888/buck,romanoid/buck,dsyang/buck,Addepar/buck,stuhood/buck,davido/buck,lukw00/buck,robbertvanginkel/buck,robbertvanginkel/buck,SeleniumHQ/buck,kageiit/buck,shybovycha/buck,vschs007/buck,Addepar/buck,stuhood/buck,facebook/buck,OkBuilds/buck,mogers/buck,marcinkwiatkowski/buck,SeleniumHQ/buck,justinmuller/buck,JoelMarcey/buck,SeleniumHQ/buck,luiseduardohdbackup/buck,lukw00/buck,robbertvanginkel/buck,raviagarwal7/buck,robbertvanginkel/buck,Addepar/buck,luiseduardohdbackup/buck,JoelMarcey/buck,1yvT0s/buck,LegNeato/buck,OkBuilds/buck,daedric/buck,justinmuller/buck,robbertvanginkel/buck,darkforestzero/buck,OkBuilds/buck,robbertvanginkel/buck,stuhood/buck,ilya-klyuchnikov/buck,vschs007/buck,davido/buck,nguyentruongtho/buck,sdwilsh/buck,bocon13/buck,ilya-klyuchnikov/buck,k21/buck,bocon13/buck,JoelMarcey/buck,zhan-xiong/buck,davido/buck,rowillia/buck,clonetwin26/buck,justinmuller/buck,k21/buck,facebook/buck,Addepar/buck,dushmis/buck,grumpyjames/buck,daedric/buck,mikekap/buck,justinmuller/buck,stuhood/buck,dsyang/buck,Learn-Android-app/buck,brettwooldridge/buck,zhuxiaohao/buck,brettwooldridge/buck,JoelMarcey/buck,daedric/buck,tgummerer/buck,lukw00/buck,daedric/buck,stuhood/buck,artiya4u/buck,tgummerer/buck,zhan-xiong/buck,liuyang-li/buck,sdwilsh/buck,rhencke/buck,MarkRunWu/buck,Dominator008/buck,raviagarwal7/buck,LegNeato/buck,bocon13/buck,rmaz/buck,vine/buck,justinmuller/buck,grumpyjames/buck,illicitonion/buck,k21/buck,hgl888/buck,romanoid/buck,mnuessler/buck,vschs007/buck,Dominator008/buck,shybovycha/buck,tgummerer/buck,mogers/buck,liuyang-li/buck,Distrotech/buck,vine/buck,mogers/buck,Learn-Android-app/buck,raviagarwal7/buck,romanoid/buck,k21/buck,mikekap/buck,dpursehouse/buck,JoelMarcey/buck,JoelMarcey/buck,zhan-xiong/buck,JoelMarcey/buck,vine/buck,stuhood/buck,raviagarwal7/buck,luiseduardohdbackup/buck,darkforestzero/buck,illicitonion/buck,grumpyjames/buck,clonetwin26/buck,tgummerer/buck,luiseduardohdbackup/buck,lukw00/buck,grumpyjames/buck,OkBuilds/buck,kageiit/buck,romanoid/buck,LegNeato/buck,stuhood/buck,tgummerer/buck,illicitonion/buck,JoelMarcey/buck,Heart2009/buck,tgummerer/buck,rhencke/buck,dsyang/buck,dsyang/buck,JoelMarcey/buck,vschs007/buck,janicduplessis/buck,OkBuilds/buck,vschs007/buck,kageiit/buck,vine/buck,clonetwin26/buck,mikekap/buck,illicitonion/buck,neonichu/buck,grumpyjames/buck,clonetwin26/buck,sdwilsh/buck,daedric/buck,MarkRunWu/buck,liuyang-li/buck,rhencke/buck,rowillia/buck,rhencke/buck,Distrotech/buck,ilya-klyuchnikov/buck,vine/buck,siddhartharay007/buck,mogers/buck,Distrotech/buck,daedric/buck,neonichu/buck,OkBuilds/buck,OkBuilds/buck,daedric/buck,robbertvanginkel/buck,neonichu/buck,rowillia/buck,shybovycha/buck,dsyang/buck,justinmuller/buck,davido/buck,rowillia/buck,lukw00/buck,zhan-xiong/buck,Dominator008/buck,zhan-xiong/buck,Heart2009/buck,clonetwin26/buck,dsyang/buck,Distrotech/buck,janicduplessis/buck,mnuessler/buck,rmaz/buck,mogers/buck,lukw00/buck,shybovycha/buck,romanoid/buck,shs96c/buck,darkforestzero/buck,zhuxiaohao/buck,Heart2009/buck,zhan-xiong/buck,daedric/buck,Addepar/buck,davido/buck,dsyang/buck,grumpyjames/buck,Addepar/buck,pwz3n0/buck,dpursehouse/buck,vine/buck,justinmuller/buck,raviagarwal7/buck,mnuessler/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,janicduplessis/buck,SeleniumHQ/buck,shs96c/buck,nguyentruongtho/buck,darkforestzero/buck,artiya4u/buck,Learn-Android-app/buck,MarkRunWu/buck,facebook/buck,dpursehouse/buck,vine/buck,marcinkwiatkowski/buck,ilya-klyuchnikov/buck,SeleniumHQ/buck,illicitonion/buck,sdwilsh/buck,clonetwin26/buck,janicduplessis/buck,marcinkwiatkowski/buck,romanoid/buck,Distrotech/buck,rmaz/buck,daedric/buck,marcinkwiatkowski/buck,facebook/buck,dushmis/buck,brettwooldridge/buck,MarkRunWu/buck,clonetwin26/buck,mnuessler/buck,zpao/buck,justinmuller/buck,dsyang/buck,sdwilsh/buck,liuyang-li/buck,brettwooldridge/buck,rowillia/buck,mikekap/buck,nguyentruongtho/buck,lukw00/buck,marcinkwiatkowski/buck,Addepar/buck,Dominator008/buck,dushmis/buck,shs96c/buck,Dominator008/buck,brettwooldridge/buck,brettwooldridge/buck,dpursehouse/buck,romanoid/buck,robbertvanginkel/buck,bocon13/buck,ilya-klyuchnikov/buck,brettwooldridge/buck,artiya4u/buck,vine/buck,rhencke/buck,Learn-Android-app/buck,zhan-xiong/buck,romanoid/buck,rhencke/buck,illicitonion/buck,zhuxiaohao/buck,MarkRunWu/buck,rmaz/buck,raviagarwal7/buck,1yvT0s/buck,rowillia/buck,LegNeato/buck,rmaz/buck,dushmis/buck,bocon13/buck,daedric/buck,shybovycha/buck,liuyang-li/buck,k21/buck,shs96c/buck,Learn-Android-app/buck,darkforestzero/buck,robbertvanginkel/buck,davido/buck,Heart2009/buck,janicduplessis/buck,siddhartharay007/buck,raviagarwal7/buck,marcinkwiatkowski/buck,vine/buck,brettwooldridge/buck,zpao/buck,brettwooldridge/buck,sdwilsh/buck,justinmuller/buck,shs96c/buck,LegNeato/buck,Heart2009/buck,SeleniumHQ/buck,rowillia/buck,romanoid/buck,mikekap/buck,shs96c/buck,LegNeato/buck,shs96c/buck,1yvT0s/buck,luiseduardohdbackup/buck,rhencke/buck,pwz3n0/buck,artiya4u/buck,Distrotech/buck,rmaz/buck,dushmis/buck,OkBuilds/buck,mnuessler/buck,stuhood/buck,vine/buck,zhuxiaohao/buck,shs96c/buck,marcinkwiatkowski/buck,rowillia/buck,neonichu/buck,rmaz/buck,dushmis/buck,romanoid/buck,bocon13/buck,kageiit/buck,stuhood/buck,zhuxiaohao/buck,illicitonion/buck,k21/buck,mikekap/buck,marcinkwiatkowski/buck,zpao/buck,marcinkwiatkowski/buck,zpao/buck,sdwilsh/buck,artiya4u/buck,Distrotech/buck,marcinkwiatkowski/buck,pwz3n0/buck,sdwilsh/buck,janicduplessis/buck