Where does Photos.app store its library and files?
After looking for it, I found it. Quite obvious actually, it is next to the former iPhoto Library.

~/Pictures/Photos Library.photoslibrary

Researching a little more, because both libraries report the same size, the import from iPhoto to Photos changed all original files to hardlinks.
So either library contains a pointer to one file on disk, thus saving disk space.

For a full backup, I think it is sufficient to backup ~/Pictures/Photos Library.photoslibrary.