Based off the existing GithubPulls.pm and GitlabPulls.pm plugins.
Also adds an integration test for the new 'giteapulls' input type to
the existing 'gitea' test.
When people reach out to the git repository they probably want to use
hydra from the same source.
This also removes the need for an overlay with simpler and more
performant direct use of the nixpkgs passed in. Before it was
re-importing nixpkgs.
test
Nixpkgs only contains a `hydra_unstable`, not `hydra`, package, so
adjust the default accordingly, and then override it to our package in
the separate module which does that.
