lastgenre: Use multi-valued albumartists field

In case the albumartist genre can't be found (often due to variations of
artist-combination wording issues, eg "featuring", "+", "&" and so on)
use the albumartists list field, fetch a genre for each artist
separately and concatenate them.

Author: j0j0

beetsplug/lastgenre/__init__.py

@@ -422,6 +422,19 @@ def _try_resolve_stage(stage_label: str, keep_genres, new_genres):
             elif obj.albumartist != config["va_name"].as_str():
                 new_genres = self.fetch_artist_genre(obj.albumartist)
                 stage_label = "album artist"
+                if not new_genres:
+                    self._tunelog(
+                        'No album artist genre found for "{}", '
+                        "trying multi-valued field...",
+                        obj.albumartist,
+                    )
+                    for albumartist in obj.albumartists:
+                        self._tunelog(
+                            'Fetching artist genre for "{}"', albumartist
+                        )
+                        new_genres += self.fetch_artist_genre(albumartist)
+                    if new_genres:
+                        stage_label = "multi-valued album artist"
             else:
                 # For "Various Artists", pick the most popular track genre.
                 item_genres = []