Access migratedphotolibrary that doesn't appear in Photos
I've discovered that I have an "iPhoto Library.migratedphotolibrary" that contains old photos—I can see them if I "Show Package Contents". Photos.app refuses to open this library, with error message implying that they should already be visible in Photos.app, which would make sense if they have been migrated.
The problem is, the photos do NOT appear in Photos. In fact, in this account, Photos.app contains no photos at all. How can I re-migrate this library into Photos.
(I do not have iPhoto.app, and I would prefer not to have to get it, since I plan to only use Photos.app.)
iMac 21.5″, macOS 10.15