Add support for building minecraft modpacks

pkgs/build-support/fetchFromCurseForge.nix

@@ -0,0 +1,23 @@
+{ lib, fetchurl }:
+
+with builtins;
+
+{ project, id, filename, ... }@args:
+
+let
+  # I think this is supposed to be some weak automation
+  # protection. This split "id" is simply part of the download URL.
+  #
+  # Note that if it's zero-prefixed we need to remove the zeroes. It's
+  # really an odd one...
+  a = head (match "0*([[:digit:]]+)" (substring 0 4 id));
+  b = head (match "0*([[:digit:]]+)" (substring 4 7 id));
+
+  url = "https://media.forgecdn.net/files/${a}/${b}/${filename}";
+  otherArgs = removeAttrs args [ "project" "id" "filename" ];
+
+in fetchurl (otherArgs // {
+  inherit url;
+  # Rename files to avoid names incompatible with the nix store
+  name = "${project}.jar";
+})