If a requirement is for a dependent that's already installed and that
dependency is not using a `default_formula` (which would have already
been converted from a `Requirement` to `Dependency` at this stage) then
we want to stop it killing the build.
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.