Recreate iPhoto library package from contents
It might be a simple extension problem:

Try adding .photoslibrary to the Photos Library folder name - basically try turning the folder into a type Photos will accept.

Interestingly, after having done something similar myself, after recovering my data from Backblaze, adding the extension switched the folder to a 'library' icon, but did not auto-hide the extension itself.
Applying similar logic to a folder by adding .app to its name auto-magically changes it into an 'app' [non-functioning] &amp; hides the extension.

You can fix this, if you see a similar result, by getting info on the Photos Library.photoslibrary file &amp; checking 'Hide extension'