extract systemConfiguration into its own output module

modules/outputs/initramfs.nix

@@ -9,6 +9,7 @@ let
   inherit (pkgs) runCommand;
 in
 {
+  imports = [ ./system-configuration.nix ];
   options = {
     boot.initramfs = {
       enable = mkEnableOption "initramfs";
@@ -22,14 +23,6 @@ in
           filesystem
         '';
       };
-      systemConfiguration = mkOption {
-        type = types.package;
-        description = ''
-          pkgs.systemconfig for the configured filesystem,
-          contains 'activate' and 'init' commands
-        '';
-        internal = true;
-      };
     };
   };
   config = mkIf config.boot.initramfs.enable {
@@ -53,8 +46,6 @@ in
           file /init ${pkgs.preinit}/bin/preinit 0755 0 0
           SPECIALS
         '';
-      systemConfiguration =
-        pkgs.systemconfig config.filesystem.contents;
     };
   };
 }

modules/outputs/system-configuration.nix

@@ -0,0 +1,28 @@
+{
+  config
+, pkgs
+, lib
+, ...
+}:
+let
+  inherit (lib) mkEnableOption mkOption mkIf types;
+  inherit (pkgs) runCommand;
+in
+{
+  options = {
+    system.outputs = {
+      systemConfiguration = mkOption {
+        type = types.package;
+        description = ''
+          pkgs.systemconfig for the configured filesystem,
+          contains 'activate' and 'init' commands
+        '';
+        internal = true;
+      };
+    };
+  };
+  config = {
+    system.outputs.systemConfiguration =
+        pkgs.systemconfig config.filesystem.contents;
+  };
+}