With Photos V 8.0 you can import an iPhoto library but not "migrate" it. You can also use the paid version of PowerPhotos to import an iPhoto library into a Photos library but will get more of the organization effort in the iPhoto library. Here's a comparison of the two apps:
If you have a lot of organizational effort in your Aperture library you might want to use the paid version of PowerPhotos to merge the Aperture library into your current Photos library.
I ran a number of tests with a 50 photo Photos library with keywords, captions, keywords and locations. I merged it into an empty library using Photos and PowerPhotos. This is what I found was imported by each;
PowerPhotos: Photos:
Albums -----------
Captions Captions
Titles Titles
Original images Original images
Keywords Keywords
Edited images Edited images
--------------- Locations
Favorites Favorites
Can detect and exclude Duplicates
duplicates or not
Neither method included projects.
Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library.