feat: use nixos-extra-modules

nix/packages.nix

@@ -1,5 +1,17 @@
-{ self, ... }:
+{ self, inputs, ... }:
 {
+  imports = [
+    (
+      { lib, flake-parts-lib, ... }:
+      flake-parts-lib.mkTransposedPerSystemModule {
+        name = "pkgs";
+        file = ./packages.nix;
+        option = lib.mkOption {
+          type = lib.types.unspecified;
+        };
+      }
+    )
+  ];
   flake = _:
     let
       inherit (self.outputs) lib;
@@ -7,4 +19,17 @@
     {
       packages = lib.swarselsystems.forEachLinuxSystem (pkgs: import "${self}/pkgs" { inherit self lib pkgs; });
     };
+
+  perSystem = { pkgs, system, ... }:
+    {
+      # see https://flake.parts/module-arguments.html?highlight=modulewith#persystem-module-parameters
+      _module.args.pkgs = import inputs.nixpkgs {
+        inherit system;
+        config.allowUnfree = true;
+        overlays = [
+          self.overlays.default
+        ];
+      };
+      inherit pkgs;
+    };
 }