gnome3: easier to override packages

pkgs/desktops/gnome-3/3.16/default.nix

···
-{ callPackage, pkgs, self }:
+{ pkgs }:
+
+let
 
-rec {
+  pkgsFun = overrides:
+    let 
+      self = self_ // overrides;
+      self_ = with self; {
+
+  overridePackages = f: pkgsFun (f self);
+
+  callPackage = pkgs.newScope self;
+
   corePackages = with gnome3; [
     pkgs.desktop_file_utils pkgs.ibus
     pkgs.shared_mime_info # for update-mime-database
···
 
   gtkhtml = callPackage ./misc/gtkhtml { };
 
-}
+    };
+  in self; # pkgsFun
+
+in pkgsFun {}

pkgs/top-level/all-packages.nix

···
     inherit (pkgs) libsoup libwnck gtk_doc gnome_doc_utils;
   };
 
-  gnome3_16 = recurseIntoAttrs (callPackage ../desktops/gnome-3/3.16 {
-    callPackage = pkgs.newScope pkgs.gnome3_16;
-    self = pkgs.gnome3_16;
-  });
+  gnome3_16 = recurseIntoAttrs (callPackage ../desktops/gnome-3/3.16 { });
 
   gnome3 = gnome3_16;