refactor: Fix a variety of filepaths for repo relayouting

This fixes readTree and the various project builds, as well
as (hopefully) most documentation links inside of the projects.

overrides/elmPackages/default.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.third_party.fetchFromGitHub {
+  owner = "NixOS";
+  repo = "nixpkgs";
+  rev = "14f9ee66e63077539252f8b4550049381a082518";
+  sha256 = "1wn7nmb1cqfk2j91l3rwc6yhimfkzxprb8wknw5wi57yhq9m6lv1";
+}) {}).elmPackages