We can't just call factory here, because the keg might not map to an existing formula (for example, a custom install, or something installed via URL, or something from a tap that has been untapped, a removed formula, etc), and `brew link` has to work for that case.

That's why, further down, we have a `keg_only?` method that handles that case, rather than trying to call `keg_only?` on a formula instance.