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.
It is important to keep the version as parsed by `builtins.parseDrvName`
monotonic for packages that often switch between stable and unstable versions,
for nix-env to be able to update them reliably.
Let’s optionally use the version format described by RFC 107:
https://github.com/NixOS/rfcs/pull/107
The downside is that it requires fetching
more commits to be able to determine the latest tag.
When `fetchFromGitHub` uses `fetchzip` instead of `fetchgit` internally,
which is the most common use case, there is no `src.url` attribute
so the update will fail.
Let’s fix that by falling back to `src.meta.homepage`,
which these fetchers set to the repository URL.
