Just one more thing...
It turns out that all of the errors that I'm referring to here occur while compiling local SPM packages. I'm beginning to think that this is:
(a) yet another bug with the Xcode implementation of SPM. (If so, it's the fourth [or so; hard to keep count] that I've uncovered.
(b) another reason to give up on SPM as a good idea whose implementation is fatally flawed.
Honestly, I've invested months on refactoring my entire development environment to use SPM for local packages, and I am about ready to chuck it. I will waste less time by undoing my reliance on SPM than I will trying to get things to work with Xcode.