This allows us to do more of the data loading that might fail safely in the
mopidy.ext module instead of having things spread all over the place.
Note that only minimal changes have been made to __main__ to make things work.
Further refactoring should follow.
As far as I know, all extensions except Mopidy-Arcam (not currently
maintained) and Mopidy-VKontakte already use the new API. Both of the
remaining extensions got open pull requests with the needed changes to use the
new API.