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
There is an overlay for the `hydra` name, but `hydra_unstable` was used, which can refer to the nixpkgs package and lead to and outdated hydra version and requires configuring the correct package attribute downstream.
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.
