modules.nix: Generate the extra argument set from the configuration

This allows for module arguments to be handled modularly, in particular
allowing the nixpkgs module to handle the nixpkgs import internally.
This creates the __internal option namespace, which should only be added
to by the module system itself.

nixos/modules/security/pam.nix

@@ -6,8 +6,9 @@
 with lib;
 
 let
+  parentConfig = config;
 
-  pamOpts = args: {
+  pamOpts = { config, name, ... }: let cfg = config; in let config = parentConfig; in {
 
     options = {
 
@@ -180,8 +181,8 @@ let
 
     };
 
-    config = let cfg = args.config; in {
-      name = mkDefault args.name;
+    config = {
+      name = mkDefault name;
       setLoginUid = mkDefault cfg.startSession;
       limits = mkDefault config.security.pam.loginLimits;