Libation never had a copyright of its own to fall back on: it only cleaned up
whatever the .aaxc file already embedded. Widevine downloads carry no cprt at
all, so they came out blank.
The copyright line is only returned by the product_details response group, so
request it on the full library scan, store it on Book, and use it as the tag
fallback. Existing libraries pick it up on their next scan, and Audible still
returns null for plenty of titles.
Co-authored-by: rmcrackan <rmcrackan@gmail.com>
