Revert "formatter: Use mkTransposedPerSystemModule"

This reverts commit e7da962abb.

modules/formatter.nix

@@ -1,21 +1,55 @@
 { config, lib, flake-parts-lib, ... }:
 let
   inherit (lib)
+    filterAttrs
+    mapAttrs
     mkOption
+    optionalAttrs
     types
     ;
   inherit (flake-parts-lib)
-    mkTransposedPerSystemModule
+    mkSubmoduleOptions
+    mkPerSystemOption
     ;
 in
-mkTransposedPerSystemModule {
-  name = "formatter";
-  option = 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).
-    '';
+{
+  options = {
+    flake = mkSubmoduleOptions {
+      formatter = mkOption {
+        type = types.lazyAttrsOf types.package;
+        default = { };
+        description = ''
+          Per system package used by <literal>nix fmt</literal>.
+        '';
+      };
+    };
+
+    perSystem = mkPerSystemOption ({ config, ... }: {
+      _file = ./formatter.nix;
+      options = {
+        formatter = mkOption {
+          type = types.nullOr types.package;
+          default = null;
+          description = ''
+            A package used by <literal>nix fmt</literal>.
+          '';
+        };
+      };
+    });
+  };
+  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 = ./formatter.nix;
 }