pkgs/top-level: expose current overlays in pkgs

This enables inspection of the currently used overlays. Useful for
usecases where nixpkgs is imported multiple times.
eg. different channels

	self: super:
	let
	  latest = import <nixpkgs-trunk> {
	    inherit (super) config overlays;
	  };
	in
	{
	  hello-custom-latest = latest.hello-custom;
	}

pkgs/top-level/all-packages.nix

@@ -5,7 +5,7 @@
  * to merges. Please use the full-text search of your editor. ;)
  * Hint: ### starts category names.
  */
-{ lib, noSysDirs, config}:
+{ lib, noSysDirs, config, overlays }:
 res: pkgs: super:
 
 with pkgs;
@@ -61,7 +61,7 @@ in
 
 
   ### Helper functions.
-  inherit lib config;
+  inherit lib config overlays;
 
   inherit (lib) lowPrio hiPrio appendToName makeOverridable;
 

pkgs/top-level/stage.nix

@@ -90,7 +90,7 @@ let
 
   allPackages = self: super:
     let res = import ./all-packages.nix
-      { inherit lib noSysDirs config; }
+      { inherit lib noSysDirs config overlays; }
       res self super;
     in res;