Forge apparently removed all `.pack.xz` files without warning.
It broke a bunch of stuff, as always. But it also means we don't need some ugly code anymore.
This is removed:
- Support for 'forge-pack-xz' and the forge-specific file download compression.
- The pack200 library we no longer need.
This stays:
- The LZMA decompression library - we may still want to use it.