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.

read-tree.nix

@@ -1,4 +1,7 @@
-initPath: { pkgs, ... } @ args:
+# TODO(tazjin): if there's a default.nix, keep traversing but don't import .nix files?
+# TODO(tazjin): avoid {} by only calling functions *after* checking what they are
+
+args: initPath:
 
 let
   inherit (builtins)