Move config bits an pieces into mopidy.config sub-modules.

Things missing before merge:

- [ ] Move load and validate into ``mopidy.config`` sub-module
- [ ] Check if we want to set ``__all__`` in ``mopidy.config`` or the sub-modules we ``*`` import from.
- [ ] Lock in naming and module splits for now, though we will encourage extension devs to only use ``mopidy.config``, so we can change our minds if we want.