Support relative paths in registries

[cstjean]

Since registries and local packages often live together, it would be nice if relative paths were supported (eg. repo = "../MyPackage.git"). At the moment, that yields ERROR: failed to clone from ../MyPackage.git, error: GitError(Code:ERROR, Class:Net, unsupported URL protocol)

At my company, we have packages on a shared drive, and would like to have a registry, but because the mount point is different on different machines, absolute paths do not work.