Make `Library/Homebrew` the new root now that *all* implementation files
are in this directory. Update filters/groups accordingly.
Note that paths in filters/groups are not anchored at the root and are
always matched against the full path. Our rules are effectively anchored
because they always include the `/Homebrew/` path component.
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.