refactor: Introduce new layout with nixpkgs in third_party

This is not the final layout yet, but makes it so that my top-level
attribute set is no longer overlaid into nixpkgs itself.

This is useful for other people who are importing my monorepo.

overrides/lispPackages/default.nix

@@ -1,8 +1,8 @@
 # One of Gemma's dependencies is missing in nixpkgs' Quicklisp
 # package set, it is overlaid locally here.
-{ pkgs, upstream, ... }:
+{ pkgs, ... }:
 
 import ./quicklisp.nix {
   inherit (pkgs) lib;
-  inherit (upstream) lispPackages;
+  inherit (pkgs.third_party.nixpkgs) lispPackages;
 }