Normalize wheel_download_dir passed to RequirementSet
Normalize wheel_download_dir passed to RequirementSet

This handles `wheel_dir = ~/.cache/pip/wheelhouse` correctly when
invoked via `pip wheel -r requirements.txt`.

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