Deal with build group changes

[msimacek]

There are upcoming changes in build group in rawhide (removal of gcc). Koschei currently kinda assumes that build group is immutable, so when it changes, it will cause incorrect resolutions as builds that were still done with gcc in build group might be resolved without it or vice versa.
I think we should either:

• Get the build group from koji at the given history point. It has event id, so it should be possible to get it at repo's create_event (but might be one-off, I'd need to read the code).
• Parse it from comps.xml. We already download those (because dnf needs it). That would be closer to what koji builders actually do.

@mizdebsk WDYT?

[mizdebsk]

Group listings are versioned, so we could definitely take advantage of this.

[msimacek]

Which way do you think is better?

[mizdebsk]

I think it would be better to use Koji Hub API to get historical group contents (option nr 1).