This ensures that all nested Git repositories (e.g. taps) are also
deleted. Previously this wasn't needed as we could just rely on the
symlinks in `Library/Formula` being missing but since Xu's good work
this inadvertedly made the bot's `brew uses foo` start showing things
from taps we hadn't previously deleted.
We're (finally) working on a documented public API for Homebrew. It's currently a work in progress; a bunch of public stuff is documented and a bunch of private stuff is undocumented. Sorry about that!
The main class you should look at is {Formula}. Assume everything else is private for now.