`nimble develop` should use canonical package name instead of git repo name

[arnetheduck]

The same package might have multiple sources (several git repositories, local repos etc) - as such, when running nimble develop --withDependencies and creating packages in vendor/, the name of the folder should reflect the canonical name of the package.

This also requires treating one of the sources as "canonical" because this will become git origin while the other sources (if they are also git-based), could be added as separate remotes.