cmd/gb-vendor/vendor: fix windows path handling
cmd/gb-vendor/vendor: fix windows path handling

Fixes #245

When walking windows paths, convert the constructed directory to a
unix style import path so it matches FindMissing later on.

mopemope/gb,URXtech/gb,inconshreveable/gb,ches/gb,tbruyelle/gb,Luit/gb,ches/gb,davidkaya/gb,hyper-carrot/gb,davidkaya/gb,mopemope/gb,lparkes/gb,MerlinDMC/gb,guiquanz/gb,tianon/gb,tbruyelle/gb,inconshreveable/gb,constabulary/gb,constabulary/gb,constabulary/gb,acasajus/gb,hyper-carrot/gb,tianon/gb,mopemope/gb