Add setFunctionArgs lib function.

Among other things, this will allow *2nix tools to output plain data
while still being composable with the traditional
callPackage/.override interfaces.

lib/modules.nix

@@ -155,7 +155,7 @@ rec {
       # a module will resolve strictly the attributes used as argument but
       # not their values.  The values are forwarding the result of the
       # evaluation of the option.
-      requiredArgs = builtins.attrNames (builtins.functionArgs f);
+      requiredArgs = builtins.attrNames (lib.functionArgs f);
       context = name: ''while evaluating the module argument `${name}' in "${key}":'';
       extraArgs = builtins.listToAttrs (map (name: {
         inherit name;