cmd/gb-vendor/vendor: add ssh as a secure scheme
cmd/gb-vendor/vendor: add ssh as a secure scheme

Also return error when scheme is unknown.

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