NixOS modules: Add error context on module arguments evaluation.

lib/tests/modules/define-_module-args-custom.nix

@@ -0,0 +1,7 @@
+{ lib, ... }:
+
+{
+  config = {
+    _module.args.custom = true;
+  };
+}

lib/tests/modules/define-enable-with-custom-arg.nix

@@ -2,7 +2,6 @@
 
 {
   config = {
-    _module.args.custom = true;
     enable = custom;
   };
 }

lib/tests/modules/import-custom-arg.nix

@@ -0,0 +1,6 @@
+{ lib, custom, ... }:
+
+{
+  imports = []
+  ++ lib.optional custom ./define-enable-force.nix;
+}