aliases: add skip aliases config

You can turn on this config option if you want to find references to
aliases in Nixpkgs. Ideally these can be removed from Nixpkgs and
eventually we can remove the alias altogether.

pkgs/top-level/stage.nix

@@ -97,7 +97,8 @@ let
       res self;
     in res;
 
-  aliases = self: super: import ./aliases.nix super;
+  aliases = self: super: if config.skipAliases or false then {}
+                         else import ./aliases.nix super;
 
   # stdenvOverrides is used to avoid having multiple of versions
   # of certain dependencies that were used in bootstrapping the