Add collection transfer logic to beatmap import-as-update flow

[peppy]

• If an updated difficulty is in a collection, its hash will be removed and replaced by the updated one (assuming difficulty name matches.
• If all difficulties in an updated beatmap set were in a collection, all difficulties post-update will be added to the same collection. This handles the case where a user has added a beatmap set to a collection (which is possible from the set header context menu).
• Depends on Move beatmap collections to realm #19430

Closes #19346

And to answer a question asked elsewhere "why was it necessary to move collections to realm to make this work?", the only answer I have is "i didn't want to pass a CollectionManager in to BeatmapImporter". Also because the deeper I got in beatmap collection code, the more I felt it required a refactor pass.

[smoogipoo]

To confirm, the editor will at some point be updated to use the ImportAsUpdate flow or something? Because beatmaps should probably be preserved in the collections they're in after being edited.

[peppy]

Yeah, that sounds like a good direction (at very least having BeatmapManager.Save() call the transfer function).

[smoogipoo]

Not sure how to feel about this being done inside this nested context as opposed to realm.Run(r => r.Refresh()); like all the other methods (on master).

But at the end of the day this is still being run on the same context as the former, so I'll let it slide.

[peppy]

I figured this was safer because as you say, the context might be different otherwise (this ensures it definitely isn't).