Picard provides a tool to look CDs up on musicbrainz ("Lookup CD" button in the main interface) but also to add new disc IDs to MB's database.  This tool uses python-discid which is not currently a requirement of the picard package, such as the button is disabled by default.  I personally went and looked in the source code to find which package was needed.  I think most people would rather have the CD lookup feature enabled by default, so I would suggest adding python-discid as a requirement to picard.