`pkgdev bugs`: support easy splitting of keywording/stabilization requests by package

[mgorny]

Currently, pkgdev bugs defaults to merging dependency packages into the main keywording/stabilization bug. Usually, this makes sense.

However, for larger keywordings it would be useful to have an option to create a bunch of separate bugs, each with explicit list of arches, so that pkgdev tatt can process them separately. Otherwise, we end up with a very long pkgdev tatt run which fails due to a single package failing and needs to be rerun entirely.