Minor cleanups in req_set.
Minor cleanups in req_set.

 - reduce the scope for some variables
 - stop accessing private member of req_to_install
   the cleanup code is idempotent and safe to call multiple times,
   so we only need to decide *if* we want to cleanup, not whether
   the requirement needs it done.

xavfernandez/pip,haridsv/pip,mindw/pip,sigmavirus24/pip,alex/pip,zorosteven/pip,xavfernandez/pip,wkeyword/pip,erikrose/pip,Gabriel439/pip,haridsv/pip,KarelJakubec/pip,ChristopherHogan/pip,pypa/pip,natefoo/pip,mindw/pip,pypa/pip,alex/pip,rbtcollins/pip,squidsoup/pip,rouge8/pip,fiber-space/pip,tdsmith/pip,rouge8/pip,habnabit/pip,yati-sagade/pip,zvezdan/pip,caosmo/pip,qbdsoft/pip,techtonik/pip,supriyantomaftuh/pip,KarelJakubec/pip,sigmavirus24/pip,fiber-space/pip,caosmo/pip,Gabriel439/pip,ChristopherHogan/pip,habnabit/pip,jasonkying/pip,squidsoup/pip,zorosteven/pip,zorosteven/pip,wkeyword/pip,KarelJakubec/pip,mindw/pip,zenlambda/pip,natefoo/pip,caosmo/pip,wkeyword/pip,sigmavirus24/pip,ChristopherHogan/pip,luzfcb/pip,dstufft/pip,RonnyPfannschmidt/pip,benesch/pip,harrisonfeng/pip,ncoghlan/pip,willingc/pip,ncoghlan/pip,zenlambda/pip,mujiansu/pip,techtonik/pip,James-Firth/pip,dstufft/pip,RonnyPfannschmidt/pip,mujiansu/pip,pfmoore/pip,benesch/pip,zvezdan/pip,willingc/pip,graingert/pip,harrisonfeng/pip,jmagnusson/pip,ncoghlan/pip,jamezpolley/pip,luzfcb/pip,mujiansu/pip,erikrose/pip,xavfernandez/pip,jasonkying/pip,harrisonfeng/pip,alex/pip,davidovich/pip,yati-sagade/pip,davidovich/pip,pfmoore/pip,RonnyPfannschmidt/pip,graingert/pip,atdaemon/pip,erikrose/pip,pradyunsg/pip,jamezpolley/pip,h4ck3rm1k3/pip,esc/pip,chaoallsome/pip,chaoallsome/pip,blarghmatey/pip,blarghmatey/pip,luzfcb/pip,nthall/pip,chaoallsome/pip,James-Firth/pip,esc/pip,graingert/pip,jmagnusson/pip,willingc/pip,jythontools/pip,atdaemon/pip,prasaianooz/pip,prasaianooz/pip,rbtcollins/pip,h4ck3rm1k3/pip,techtonik/pip,zenlambda/pip,nthall/pip,sbidoul/pip,prasaianooz/pip,habnabit/pip,benesch/pip,fiber-space/pip,supriyantomaftuh/pip,pradyunsg/pip,supriyantomaftuh/pip,dstufft/pip,jythontools/pip,tdsmith/pip,esc/pip,sbidoul/pip,natefoo/pip,squidsoup/pip,nthall/pip,jasonkying/pip,qbdsoft/pip,yati-sagade/pip,jamezpolley/pip,James-Firth/pip,rbtcollins/pip,h4ck3rm1k3/pip,jythontools/pip,davidovich/pip,Gabriel439/pip,atdaemon/pip,tdsmith/pip,blarghmatey/pip,rouge8/pip,haridsv/pip,jmagnusson/pip,qbdsoft/pip,zvezdan/pip