redesign module hierarchy

nix/snow/flake/modules/formatter.nix

@@ -1,52 +1,26 @@
-{ config, lib, flake-parts-lib, ... }:
-let
-  inherit (lib)
-    filterAttrs
-    mapAttrs
+{
+  lib,
+  snow,
+  ...
+}: let
+  inherit
+    (lib)
     mkOption
-    optionalAttrs
     types
     ;
-  inherit (flake-parts-lib)
-    mkPerSystemOption
+  inherit
+    (snow)
+    mkPerSystemFlakeOutput
     ;
 in
-{
-  options = {
-    flake.formatter = mkOption {
-      type = types.lazyAttrsOf types.package;
-      default = { };
+  mkPerSystemFlakeOutput {
+    name = "formatter";
+    option = mkOption {
+      type = types.nullOr types.package;
+      default = null;
       description = ''
-        An attribute set of per system a package used by [`nix fmt`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html).
+        A package used by [`nix fmt`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html).
       '';
     };
-
-    perSystem = mkPerSystemOption {
-      _file = ./formatter.nix;
-      options = {
-        formatter = mkOption {
-          type = types.nullOr types.package;
-          default = null;
-          description = ''
-            A package used by [`nix fmt`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html).
-          '';
-        };
-      };
-    };
-  };
-  config = {
-    flake.formatter =
-      mapAttrs
-        (k: v: v.formatter)
-        (filterAttrs
-          (k: v: v.formatter != null)
-          config.allSystems
-        );
-
-    perInput = system: flake:
-      optionalAttrs (flake?formatter.${system}) {
-        formatter = flake.formatter.${system};
-      };
-
-  };
-}
+    file = ./apps.nix;
+  }