Update game data storage scheme

Instead of current `hash.txt`, let's better move the game files into a hashed directory. This will allow for better updates in case the hash changes, or the user wants to have several content packs with different hashes.

Start by locating the code that reads and writes `hash.txt`, then trace how game files and content-pack hashes are currently stored. Define how hashed directories should support hash changes and multiple content packs, and verify that updates and distinct hashes remain separated.

Written by the indexing model from the issue text.