manifest: do not gather changelogs if they will not be used
manifest: do not gather changelogs if they will not be used

I wanted to keep the data collection and serialization parts completely
separate, and I thought that gathering of the changelogs is insignificant.
But with a lot of packages, it can add up. So let's do it only if it'll
be useful later on.

A direct motivation is https://bugzilla.redhat.com/show_bug.cgi?id=2002038:
rpm gets confused by some package names and cannot show the changelog.

systemd/mkosi,systemd/mkosi