+1 to this, although I'm not sure if we should create krbV, librepo packages. Probably should have some better names instead?

I would name them based on intent... Like you don't want imagefactory, but you want builder to be able to build this specific image type like container.

I think it's a good idea to split out imagefactory or livemedia, but it's not clear to me why we would split out the others at this time.

I've gone through optional imports in kojid and this is the simple result.
krbV vs gssapi - first one is available only in py2 installations, second in both, so yes, here it doesn't make much sense (it is already required in respective packages)
librepo should be tied to dnf support. It is not about splitting, problem is, that nothing of these is required via koji-builder, so you can end e.g. in situation, that you set mock.package_manager='dnf', but it is not installed anywhere. It would be easier to me to say, I've f30 channel on RHEL7 and it means, koji-builder-dnf must be installed on those builders (instead of explicitly listing librepo + dnf as a separate rpms in ansible playbook).

koji-builder-dnf (librepo, dnf)
koji-builder-image (imagefactory) for image building
koji-builder-livemedia (pykickstart, pycdio) for livecd, livemedia, appliance

Hard to say, how the latter should be called better.

librepo is used regardless of yum or dnf backend, so I'd consider it core at this point.