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/lispPackages/default.nix

@@ -0,0 +1,8 @@
+# One of Gemma's dependencies is missing in nixpkgs' Quicklisp
+# package set, it is overlaid locally here.
+{ pkgs, upstream, ... }:
+
+import ./quicklisp.nix {
+  inherit (pkgs) lib;
+  inherit (upstream) lispPackages;
+}