The effect would be much more obvious if the modules did not depend on each other. In cases like this where they do, SwiftPM will have already put the lower‐level one in the right place so that it works by accident anyway. But increasingly SwiftPM’s isn’t the only thing using the manifest. A lot of accidentally working things suddenly broke when Xcode added support for packages, because it went about the build process its own way and things no longer lined up. I’ve learned the hard way not to rely on anything that appears to work with SwiftPM, but lacks any official documentation saying it is an intentionally supported set‐up.
I probably will eventually, but I don’t know when I will get to it.