This change converts redundant attribute assignments of the form `a =
a;` or `a = someSet.a;` into cleaner `inherit` statements. This reduces
verbosity and follows common Nix style for bringing attributes into
scope.
Statix Codes: W03 (manual_inherit), W04 (manual_inherit_from)
Also include statix and the rule in our configuration.
We have stuff that's referenced in CI but not available as flake outputs
to build. Adding default.nix outputs to flake.nix so flake users can
reference them easier.
