Add a not-working snippet to show where system packages are imported.

nix/configuration/configuration.nix

@@ -198,6 +198,12 @@
     in
     formatted;
 
+  # environment.etc."system-packages-with-source".text = builtins.concatStringsSep "\n\n" (
+  #   builtins.map (
+  #     x: x.file + "\n" + builtins.concatStringsSep "\n" (builtins.map (s: "  " + s) x.value)
+  #   ) config.environment.systemPackages.definitionsWithLocations
+  # );
+
   # nixpkgs.overlays = [
   #   (final: prev: {
   #     nix = pkgs-unstable.nix;