Now, nothing touches the configuration apart from `OsuGame`, making
everything else flow better and avoid weird cyclic set bugs.
Closes https://github.com/ppy/osu/issues/20234.
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.