If apr is installed, we still want to grab it during dependency
resolution in case we need to pass options to the generated dependency.
In other words, it doesn't make sense for the satisfiable condition to
include the default formula, as the default formula is the fallback for
when the requirement is *not* satisfied.
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.