Restore album_for_id and track_for_id functions in metadata_plugins to
support data source-specific lookups. These functions accept both an ID
and data_source parameter, enabling plugins like mbsync and missing to
retrieve metadata from the correct source.
Update mbsync and missing plugins to use the restored functions with
explicit data_source parameters. Add data_source validation to prevent
lookups when the source is not specified.
Add get_metadata_source helper function to retrieve plugins by their
data_source name, cached for performance.
These functions were used to generate mock data for tests but have been
replaced with direct instantiation of AlbumInfo and TrackInfo objects.
This change simplifies the test code and removes unnecessary helper
functions.
External Python packages interfacing beets may want to use an in-memory
beets library instance for testing beets-related code.
The `TestHelper` class is very helpful for this purpose.
Previously `TestHelper` was located in the `test/` directory.
Now it is part of `beets` itself (`beets.test.helper.TestHelper`) and
can be easily imported.
