Importer metadata source is set as a field

TrackInfo and AlbumInfo were already keeping track of this,
so just had to add it as an actual field to Item and Album

See #1311

beets/autotag/__init__.py

@@ -42,6 +42,8 @@ def apply_item_metadata(item, track_info):
     item.mb_trackid = track_info.track_id
     if track_info.artist_id:
         item.mb_artistid = track_info.artist_id
+    if track_info.data_source:
+        item.data_source = track_info.data_source
     # At the moment, the other metadata is left intact (including album
     # and track number). Perhaps these should be emptied?
 
@@ -125,7 +127,8 @@ def apply_metadata(album_info, mapping):
                       'language',
                       'country',
                       'albumstatus',
-                      'albumdisambig'):
+                      'albumdisambig',
+                      'data_source',):
             value = getattr(album_info, field)
             if value is not None:
                 item[field] = value

beets/autotag/hooks.py

@@ -137,6 +137,8 @@ class TrackInfo(object):
     - ``artist_sort``: name of the track artist for sorting
     - ``disctitle``: name of the individual medium (subtitle)
     - ``artist_credit``: Recording-specific artist name
+    - ``data_source``: The original data source (MusicBrainz, Discogs, etc.)
+    - ``data_url``: The data source release URL.
 
     Only ``title`` and ``track_id`` are required. The rest of the fields
     may be None. The indices ``index``, ``medium``, and ``medium_index``

beets/autotag/mb.py

@@ -161,6 +161,7 @@ def track_info(recording, index=None, medium=None, medium_index=None,
         medium=medium,
         medium_index=medium_index,
         medium_total=medium_total,
+        data_source='MusicBrainz',
         data_url=track_url(recording['id']),
     )
 

beets/library.py

@@ -388,6 +388,8 @@ class Item(LibModel):
         'channels':    types.INTEGER,
         'mtime':       DateType(),
         'added':       DateType(),
+
+        'data_source': types.STRING,
     }
 
     _search_fields = ('artist', 'title', 'comments',
@@ -790,6 +792,8 @@ class Album(LibModel):
         'original_year':      types.PaddedInt(4),
         'original_month':     types.PaddedInt(2),
         'original_day':       types.PaddedInt(2),
+
+        'data_source':        types.STRING,
     }
 
     _search_fields = ('album', 'albumartist', 'genre')
@@ -832,6 +836,7 @@ class Album(LibModel):
         'original_year',
         'original_month',
         'original_day',
+        'data_source',
     ]
     """List of keys that are set on an album's items.
     """