Always use syspath conversions (#3690 split up, part 3)

[wisp3rwind]

Follow-up to #4830 and #4848:

I've had #3690 in the pipeline for a long time; and it has grown too big to look at in one piece. This is just the first commit rebased on master. It is still quite big, but only touches test code, so should be safe (assuming CI is green).

This is part 3, with some changes to beets' core. Thus, this is probably the most sensitive PR in the series.

[sampsyo]

Excellent! I can't see anything wrong here—it is comforting to know that syspath is (supposed to be) idempotent, so it's unlikely to break things too bad by calling it unnecessarily. (And anyway, all of these seem to me to qualify as "necessary.")