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

Things missing before merge:

- [x] Move load and validate into ``mopidy.config`` sub-module
- [x] Add tests for these parts of the config system.
- [x] Check if we want to set ``__all__`` in ``mopidy.config`` or the sub-modules we ``*`` import from.
- [x] 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.

Switching to reading config as bytes is not part of this PR.