- redefine `{default,shell}.nix` to derive from `flake.nix` (using
`flake-compat`)
- use `flake-parts`' `_module.args.pkgs` machinery to apply overlays
(instead of `pkgsFor`)
- extract `homeManagerModule` to `nix/module.nix`
- rename package at `nix/default.nix` to `nix/package.nix`
Co-authored-by: reo101 <pavel.atanasov2001@gmail.com>
