`Library/ENV` like `Library/Homebrew` is part of Homebrew basecode. It
should be able to be accessed during the `brew tests`.
By adding HOMEBREW_ENV_PATH variable, we allow test suit to locate these
codes.
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.