lib/default.nix: improve error reporting in imported modules

The hidden `_file` attribute allows more informative error reporting when evaluation fails in the module system. This should no longer blame the `lib.mkModules` function when an error happens in my modules.

lib/default.nix

@@ -18,7 +18,9 @@
         imported = import mod;
       in {
         # args = { config, lib, modulesPath, options, pkgs, ... }
-        __functor = _: args: imported (args // extraArgs);
+        __functor = _: args: (imported (args // extraArgs)) // {
+            _file = mod; # better error reporting in the module system
+          };
         __functionArgs = final.functionArgs imported;
       }))
     ];