Consolidate upgrade logic somewhat.
Consolidate upgrade logic somewhat.

We had several overly-scope variables in prepare_files which weren't needed.

install is fully modelled by satisfied_by, if we move some logic into the right
guarded conditions, which this does.

not_found was already obsolete.

best_installed really only needed to exist and be checked within the
index-consulting upgrade block.

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