a67950f20b added `url` attribute
from `fetchurl` and therefore also from `fetchzip`.
We previously relied on `url` from fetchgit-based fetchers
to find the repo URL but now it will just return tarballs
in the case of `fetchFrom{GitHub,GitLab}`.
Let’s add an attribute to `fetch{git,FromGitHub,FromGitLab}`
to expose a repo URL consistently.
