fix(third_party): Shuffle code around to be compatible with readTree

As per the previous commit these files were no longer being imported,
which is now fixed by moving them into separate subfolders.

third_party/nixery/default.nix

@@ -0,0 +1,18 @@
+# Technically I suppose Nixery is not a third-party program, but it's
+# outside of this repository ...
+{ pkgs, ... }:
+
+let src = pkgs.third_party.fetchFromGitHub {
+  owner = "google";
+  repo = "nixery";
+  rev = "4f6ce83f9296545d6c74321b37d18545764c8827";
+  sha256 = "19aiak1pss6vwm0fwn02827l5ir78fkqglfbdl2gchsyv3gps8bg";
+};
+in import src {
+  pkgs = pkgs.third_party;
+  preLaunch = ''
+    export USER=root
+    cachix use tazjin
+  '';
+  extraPackages = [ pkgs.third_party.cachix ];
+}