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.

tools/blog_cli/default.nix

@@ -1,6 +1,6 @@
 { pkgs, ... }:
 
-pkgs.buildGoPackage {
+pkgs.third_party.nixpkgs.buildGoPackage {
   name = "blog_cli";
   goPackagePath = "github.com/tazjin/personal/blog_cli";
   src = ./.;