haskellPackages: Avoid outer `rec { .. }`

This is much nicer for overlays. For example, if someone binds a new
compiler / package set, its integer-simple variation will automatically
appear.

pkgs/top-level/haskell-packages.nix

@@ -36,7 +36,10 @@ let
     });
   };
 
-in rec {
+  # Use this rather than `rec { ... }` below for sake of overlays.
+  inherit (pkgs.haskell) compiler packages;
+
+in {
   lib = haskellLib;
 
   compiler = {