_get_track resolved the artist with track_data.get('contributors', [track_data['artist']]),
whose default is evaluated eagerly, so track_data['artist'] raises KeyError whenever the
artist key is absent, even when contributors is present. Guard the fallback the same way
album_for_id already does. Fixes#4339.
Singleton searches built the query as `<title> artist:"<artist>"`. Deezer
discards unquoted free text as soon as a query contains any field:"value"
filter, so that was evaluated as `artist:"<artist>"` alone - every track by
the artist, in Deezer's own relevance order, truncated to `search_limit`
(default 5). Substituting nonsense for the title returns a byte-identical
result set. For any artist with more releases than that window, the track
being imported was simply never among the candidates offered.
Filtering on the title as well (`track:"<title>" artist:"<artist>"`) is not
a fix: `artist:` matches loosely enough to return unrelated artists, so the
two filters can intersect to nothing even for a correctly tagged file.
`track:"Get Lucky" artist:"Daft Punk"` returns zero results, while the plain
free text `Get Lucky Daft Punk` returns the right track first.
Measured over 12 tracks at the default `search_limit`, counting the wanted
track appearing anywhere in the results:
`<title> artist:"..."` 6/12, mean rank 1.50
`track:"..." artist:"..."` 7/12, mean rank 1.00
free text 10/12, mean rank 1.00
Album searches are unchanged; `album:"<name>"` has no equivalent problem.
Adds test/plugins/test_deezer.py, which did not exist.
