Basically, `BeatmapCarousel.CurrentSelection`, which is
magic-object-typed, can no longer use `BeatmapInfo` directly, it now
must also use `GroupedBeatmap`.
This spills out all the way into song select because of beatmap
selection flows that require hookup from song select.
Takes changes in https://github.com/ppy/osu/pull/34233 and removes *all*
of the complexity.
Supersedes and closes#34233.
Contains same caveat that we can only display one beatmap once in the
carousel; this will be addressed separately.