Nevermind, I was wrong. It's actually NOT resolved in 5.4. I just had neglected to clear my DerivedData folder after I switched one of our modules from being an Xcode project, to being a Swift package.
So yeah, SPM still can't make a Swift module that's visible to Obj. C. [SR-14575] Frameworks built from Swift packages lack a Headers folder · Issue #4430 · apple/swift-package-manager · GitHub
Do they have anyone at Apple who is actually testing SPM or working with it? Seems very much like an alpha version right now based on all the bugs we've been having with it. It's making me regret using it every day.