This is not really that simple, as @outsidecontext mentions. https://thomas.apestaart.org/thomas/trac/wiki/DAD/Rip discusses some pitfalls, but there are a lot of things to consider when extracting audio from a CD: hidden track ones, pregaps, scratches, faulty drives, drive read offsets, drive audio caching, CD DRM techniques, …
If Picard is to properly extract audio from a CD to feed to its fingerprinter, it would have to be able to deal with all of the above. Using dedicated ripping software (I like morituri 
) will likely get you better results than anything we could cram into Picard, without making Picard an actual ripper (and possibly even then).