parse_requirements: add _remove_prefixes
parse_requirements: add _remove_prefixes

this isolates the line parsing bit

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