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.

pkgs/build-support/emacs/wrapper.nix

@@ -40,7 +40,7 @@ packagesFun: # packages explicitly requested by the user
 
 let
   explicitRequires =
-    if builtins.isFunction packagesFun
+    if lib.isFunction packagesFun
       then packagesFun self
     else packagesFun;
 in