Despite updating to the most recent Xcode repeatedly, I was never able to add a swift package using Xcode -- it would get stuck at validation and never get past that. I filed a feedback (and may have asked here? can't find it) and never heard back.
Today, I managed to figure it out, by creating a project on one machine (where it worked), and checking it out on one that didn't work. And when I tried to launch Xcode on the second machine... it told me that it couldn't use swift packages with a legacy build setting.
I changed that (long, long, long set) preference, and... suddenly it worked.
In case anyone else is as stubborn and ludditious as I am, and find this useful.