Still got this problem too. Tried all the 'solutions' and still iMovie sees the old iPhoto Library and will not open the newer Photos library. I've even deleted the old iPhotos app yet iMovie still pulls the library from my photos folder on start up. I've reset iMovie still no joy. I've created new library's in Photos and made them the system library and deleted them and relinked it back to my Photos Library making that the system library again and iMovie still only shows the iPhotos Library option in the Librarys list. Getting fed up now.
An OS update would likely fix it but don't want to risk updating to High Sierra at the moment as I'm too busy and don't want to risk conflict with other apps, this is already sucking up too much of my time. Any help would be greatly appreciated.
Running El Capitan 10.11.3, iMovie 10.0.4 and Photos 1.5.
To summarise: I want to open iMovie and see my Photos Library on the left, not the older iPhotos Library.