Swift-only SwiftPM frameworks again expose the -Swift.h header to Objective-C

Swift-only SwiftPM framework targets now let Xcode synthesize their modulemap again, restoring the generated -Swift.h Objective-C compatibility header. This fixes builds and cache runs where Objective-C consumers failed to import Swift @objc classes from packages like RollbarNotifier, reporting errors such as “unknown receiver”. The change reverts a previous workaround that emitted a headerless modulemap for these targets.