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

read-tree.nix

@@ -1,4 +1,4 @@
-path: self: super:
+path: { pkgs, ... } @ args:
 
 let
   inherit (builtins)
@@ -15,8 +15,6 @@ let
     toPath
     toString;
 
-  args = { pkgs = self; };
-
   zipAttrs = names: values:
     if (names == []) || (values == [])
     then []