Add feature to include singles into a combined album directory

[fmigneault]

Potential use cases:

• given an artist that has some "singles", combine these songs into a common "Singles" album directory if --combine-singles (or similar flag) is specified
• given an artist that only has "singles" (no albums), provide an appropriate error message to provide the flag to include singles (?) or automatically include them, depending on the default resolution of the default value for the option flag.
  This requires handing of the following line since it will raise AttributeError on "no albums returned".
  

  aiu/aiu/youtube.py

  Line 296 in f3a2286

  albums = meta.get("albums", {}).get("items", [])

  • Example artist that has only singles: https://music.youtube.com/channel/UC6ePNLohr4AKWSDhy6ZS8Ww
• alternatively, have the flag exclude them by default?
  The reason where this could be relevant is that singles are often released early, until they are eventually included in an album, causing unnecessary song duplicates. Therefore, leave it up to the user to decide if explicit duplicates (or extra song to fetch) should be done.
• Implementation wise, adding additional aiu.main calls for singles is equivalent to calling the following for each song:
  aiu --link <single-link> --outdir "<artist-prefix>/Singles"
  Therefore, enabling this by default (or not) is relatively not much extra work.

[fmigneault]

YouTube Music started returning (sporadically) singles_&_eps listing, on top of the albums that the tool is using. Therefore, we might have a truncated list missing the EPs that were previously returned. However, sometimes, YouTube Music still returns them all under albums...

If integrating a feature with singles handling, it will have to split out potential EPs as well, making it somewhat more complicated to manage, but relevant to get the sometimes-missing EPs.