Allow `Cerulean.mkNexus` to be an alias for `flake-parts.lib.mkFlake`
also rename `Cerulean` to `cerulean` in Nix to maintain the naming convention.

Using `flake-parts` ensures Cerulean is usable without restricting
yourself only to the Cerulean ecosystem.

- [ ] extend the options.nixpkgs to allow any number of package repositories!!
- [x] auto-propagate the same specialArgs for hosts to home-manager