Argh, note to myself: Simply define $hide_packages = (!id(new PhabricatorPackagesApplication())->isInstalled()); and add ->setIsHidden($hide_packages)...

@20after4 suggested an "is-application-installed"-aware class-finder recently - I wonder if that would also handle this?

I think it might be applicable here and in several other places that follow the general pattern of “look up all subclasses of a given feature and then loop over all of them to run extension hooks “