Extract the relevant logic from `formula.rb`, moving to `keg.rb` and
then use this logic in `bundle/commands/cleanup.rb` to ensure that we
don't say we need to uninstall formulae that should be still kept.
We've had requests for this in Homebrew/bundle a few times so let's
implement it both for there and for `brew edit`.
Tested on my machine with `cursor` and working as expected.
