Extends `find_exact` and `search` in mopidy.backends.local.library
to support the `filename` query field. This field can get passed in
from the MPD frontend and would break with a `LookupError` when used.
This patch fixes the issue and introduces two new tests to cover the
added functionality.
The frontends use the new core actor, while the core actor uses the backend.
This is a step towards supporting multiple backends, where the core actor will
coordinate the backends.
