This is different from the working beatmap's `.Beatmap` property in that it is mutated by the ruleset/editor.
So hit objects, for example, are actually of type `Slider` and such instead of the legacy `ConvertSlider`.
This should be preferred over `workingBeatmap.Beatmap`.
Makes use of the new `BeatmapSet.GetPathForFile` method and removes dependency on `WorkingBeatmap` specifically, allowing us to switch to `IWorkingBeatmap` later.