None of the solutions outlined here worked for me. The only way I got it to work was closing Xcode, deleting all the caches, then when adding my package: instead of choosing "Up to Next Major Version" as the dependency rule, I chose "Exact Version" and typed in the latest version number.
After that I could navigate to my top-level project, then on the "Package Dependencies" tab, change it to "Up to Next Major Version". That seems to work, at least for now.