nixos/nixpkgs: show definition files in config assertion

nixos/modules/misc/nixpkgs.nix

@@ -387,6 +387,9 @@ in
 
           Current value:
           ${lib.generators.toPretty { multiline = true; } cfg.config}
+
+          Defined in:
+          ${lib.concatMapStringsSep "\n" (file: "  - ${file}") opt.config.files}
         '';
       }
     ];

nixos/modules/misc/nixpkgs/test.nix

@@ -17,6 +17,7 @@ let
     nixpkgs.buildPlatform = "aarch64-linux";
   };
   externalPkgsWithConfig = {
+    _file = "ext-pkgs-config.nix";
     nixpkgs.pkgs = pkgs;
     nixpkgs.config.allowUnfree = true;
   };
@@ -122,6 +123,9 @@ lib.recurseIntoAttrs {
           {
             allowUnfree = true;
           }
+
+          Defined in:
+            - ext-pkgs-config.nix
         ''];
     assert getErrors {
         nixpkgs.localSystem = pkgs.stdenv.hostPlatform;