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.
If callers of `brew bundle check` have already output some formulae
errors, they can set the
`HOMEBREW_BUNDLE_CHECK_ALREADY_OUTPUT_FORMULAE_ERRORS` environment
variable to the names of the formulae that have already been output.
- Sort output by key so it's more readable.
- Skip exporting empty values because shell scripts treat them as unset.
- Skip exporting non-Homebrew things that were already set in the old
environment to avoid massive duplication and higher chances of e.g.
bad escapes breaking things.
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.
