extract systemConfiguration into its own output module

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;
+  };
+}