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

overrides/elmPackages.nix

@@ -0,0 +1,10 @@
+# Gemma needs an older version of Elm to be built. Updating it to
+# the newer version is a lot of effort.
+{ pkgs, ... }:
+
+(import (pkgs.fetchFromGitHub {
+  owner = "NixOS";
+  repo = "nixpkgs";
+  rev = "14f9ee66e63077539252f8b4550049381a082518";
+  sha256 = "1wn7nmb1cqfk2j91l3rwc6yhimfkzxprb8wknw5wi57yhq9m6lv1";
+}) {}).elmPackages