versions: raise NotImplemented for local/remote of a version group
versions: raise NotImplemented for local/remote of a version group

This is not a clean solution, but it should work for now. I'm not yet
sure about the interface for version groups anyway (why do they have to
behave like a source anyway?)

PolyJIT/benchbuild,PolyJIT/benchbuild,PolyJIT/benchbuild,PolyJIT/benchbuild