One of my pending work items for post-realm merge.
The lowest-level import task is no longer asynchronous, as we don't want
it to span multiple threads to allow easier interaction with realm.
Removing the `Task` spec simplifies a heap of usages.
Individual usages should decide whether they want to run the import
asynchronously, by either using an alternative override or spooling up a
thread themselves.
This is commonly used in tests in a way where it's not feasible to
guarantee correct results unless a refresh is called. This method
shouldn't really be used outside of tests anyway, but that's for a
folow-up effort.
It seems that no usages of `BeatmapSetInfo` detaches require files - a
`WorkingBeatmap` is always obtained before doing further lookups.
Therefore we can omit this include unless the detaching object is a
`BeatmapInfo`. A refetch is performed when retrieving a
`WorkingBeatmap` to complete the equation.
This paves the way for the framework code quality change
(https://github.com/ppy/osu-framework/pull/4873) which stops exposing
the constructor.
Most of the restructuring here is required to give `PreviewTrackManager`
an adjustable target to apply the global mute.