Add Deezer plugin

[rhlahuja]

Add a Deezer plugin to allow tracks and albums to be matched using the Deezer API.

Introduce a base MetadataSourcePlugin class to define shared API import logic. This will be integrated into the Spotify plugin in a followup PR.

[sampsyo]

Awesome! Looking pretty good so far.

I have just a couple of high-level questions:

• Since you said you repurposed some logic from your Spotify plugin, would it make sense to factor out some code that gets reused into a common library?
• It looks like there are also changes to the Spotify plugin here too—would you mind breaking those out into a separate PR?

[rhlahuja]

• Yeah, I was going back and forth on this. Some helpers like album_distance, track_distance, and _get_artist can easily be factored out, although some of the higher-level functions like track_for_id and album_for_id may not quite be similar enough to easily modularize. I'll take another look at this later and refactor what makes sense. Is there a particular place you'd like to place the common library? Something like beetsplug/api_utils.py?
• Sure thing -- I'll include the aforementioned Spotify refactor in a separate PR.

[sampsyo]

Cool; thanks for looking into it! Depending on the exact scope, something like beets.autotag or beets.autotag.match could be a good fit for these reusable bits.

[rhlahuja]

@sampsyo I've factored out shared logic into a base APIAutotaggerPlugin class (better name suggestions welcome). I plan to revert all changes to spotify.py and discogs.py and include them in a separate PR, but I committed them all here for now to get a sense of the final state. Before I begin testing and documenting, what are your thoughts so far?

[sampsyo]

Awesome; thank you for working on this! This looks great overall. I left a few comments inline.

[sampsyo]

In retrospect, perhaps the most natural home for this base class would be in the beets.plugins module?

In general, we have talked several times in beets's history about creating "template" base classes like this one to make it simpler to write certain kinds of plugins. So I'm very happy to see that you came to the same idea independently! It would be great to introduce more of these, so perhaps creating a new place to gather plugin templates together would be the thing to do.

[sampsyo]

Also, what do you think about the name MetadataSourcePlugin?

[rhlahuja]

Awesome! I was unsure if this class is the right approach, but it's great to hear something like this was already being considered. And I agree, beets.plugins.MetadataSourcePlugin seems like a more natural place/name for this.

[sampsyo]

We could even mention the new base class in a "for developers" section.

[sampsyo]

No need for a nested list here if it's just one bullet.

[sampsyo]

This section doesn't seem to add too much—both of these points are pretty well covered by your introductory sentence above. 😃

[rhlahuja]

@sampsyo this is tested and ready for a final review. Once merged, I'll open another PR for reusing this shared logic in the Spotify and Discogs plugins.

[sampsyo]

Nice! This looks great! I have one small comment above, but I can resolve it myself during the merge. Thank you for taking the time to make this base class awesome! ✨

[sampsyo]

I think this change can be reverted now because Distance is no longer used here.

[sampsyo]

Oops; I'm obviously wrong about that. Not changing anything!