example refactor snowflake module system

nix/snow/flake/modules/nixpkgs.nix

@@ -0,0 +1,26 @@
+#
+# Nixpkgs module. The only exception to the rule.
+#
+# Provides a `pkgs` argument in `perSystem`.
+#
+# Arguably, this shouldn't be in flake-parts, but in nixpkgs.
+# Nixpkgs could define its own module that does this, which would be
+# a more consistent UX, but for now this will do.
+#
+# The existence of this module does not mean that other flakes' logic
+# will be accepted into flake-parts, because it's against the
+# spirit of Flakes.
+#
+{
+  config = {
+    perSystem = { inputs', lib, ... }: {
+      config = {
+        _module.args.pkgs = lib.mkOptionDefault (
+          builtins.seq
+            (inputs'.nixpkgs or (throw "flake-parts: The flake does not have a `nixpkgs` input. Please add it, or set `perSystem._module.args.pkgs` yourself."))
+            inputs'.nixpkgs.legacyPackages
+        );
+      };
+    };
+  };
+}