I've been building just fine with a dozen SPM dependencies, and am now running into this. I had a merge conflict with Package.resolved (not sure why it keeps changing). I discarded it, and ran into the problem. I told Xcode 13b1 to Resolve Package Versions, but that didn't seem to do anything. So I reset the cache, and that seemed like it was working, but when it was all done, there was still one missing package. There are no errors in the resolution log. The one missing package is not in the Package.resolved file.
I just tried deleting the resolved file again. Xcode re-creates it, but it's always missing that one package, despite it being in the project list of dependencies.
I just tried removing it and re-adding it to the Xcode project. Didn't fix the resolved file, same error.
I reset it again. Now I see it's kinda skipping Marshal, and I realize it's because it's supposed to be finding my checked-out version of the package that I modified. So I poked around my project for a bit to see if it was still there, and it was. I built again, and it worked.
I have no idea what was going on, but this wasn't good.
It also brings up a question: what happens if I've taken the steps to make an SPM dependency editable in Xcode, and I commit and a colleague pulls but doesn't have the same writable package?