(Partially) refactor Nix shell configuration for new Codename setup

Codename now introduces the usage of Haxe 4.3.7 as the recommended
Haxe version, and moves a few files around, including the library
installation scripts. The Nix shell configuration has been updated
to accomodate for this.

Do note that this is an incomplete refactor, and is used for issue
reproduction as well. See CodenameCrew#769 for more information.

Author: r6915ee

shell.nix

@@ -1,9 +1,27 @@
-{ }:
+{
+  pkgs ? import <nixpkgs> {
+    overlays = [
+      (final: prev: {
+        # Haxe does not compile properly using the overlay
+        # below, due to an OCaml error. Do note that the patches
+        # have been removed as a result of an outdated patch.
+        #
+        # haxe = prev.haxe.overrideAttrs (old: {
+        #   version = "4.3.7";
+        #   src = prev.fetchgit {
+        #     url = "https://github.com/HaxeFoundation/haxe.git";
+        #     tag = "4.3.7";
+        #     hash = "sha256-sQb7MCoH2dZOvNmDQ9P0yFYrSXYOMn4FS/jlyjth39Y=";
+        #     fetchSubmodules = true;
+        #   };
+        #   patches = [ ];
+        # });
+      })
+    ];
+    config = { };
+  },
+}:
 let
-  pkgs = import (builtins.fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs/archive/c407032be28ca2236f45c49cfb2b8b3885294f7f.tar.gz";
-  }) { };
-
   libs =
     with pkgs;
     [
@@ -37,6 +55,9 @@ pkgs.mkShell {
   LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath libs;
 
   shellHook = ''
-    source update.sh
+    cd building/
+    # The shell script does not work properly with
+    # Nix shells!
+    # source setup-unix.sh
   '';
 }