Introduce a new RequestHandler base class to introduce a shared session,
centralize HTTP request management and error handling across plugins.
Key changes:
- Add RequestHandler base class with a shared/cached session
- Convert TimeoutSession to use SingletonMeta for proper resource
management
- Create LyricsRequestHandler subclass with lyrics-specific error
handling
- Update MusicBrainzAPI to inherit from RequestHandler
This PR moves the `vfs.py` module, which is only used by plugins, to
avoid polluting the main beets namespace. Also exposes the `vfs` and
`art` module from beets with a deprecation warning.
