chore(nix): Move files around to conform to new read-tree layout

Broadly speaking, the following things are included:

* there is now a uniform `args` struct that is passed to all
  derivations, package headers have been changed appropriately
* overrides are now loaded from a separate `override` folder just
  using read-tree.nix
* third-party packages have moved into the `third_party` attribute set

third_party/nixery.nix

@@ -1,6 +1,6 @@
 # Technically I suppose Nixery is not a third-party program, but it's
 # outside of this repository ...
-{ pkgs }:
+{ pkgs, ... }:
 
 let src = pkgs.fetchFromGitHub {
   owner = "google";