DocC generation now works for Swift packages with header-implied imports

tuist generate for packages with DocC builds now correctly resolves cross-target imports surfaced in public Objective-C headers (informal dependencies documented in Package.swift rather than declared), such as FirebaseCoreExtension importing FirebaseCore. Generated framework targets that need dependency module maps at the canonical Modules/module.modulemap path now expose them so Clang’s ExtractAPI can find them. This unblocks DocC builds for packages like Firebase’s iOS SDK that use header-implied in-package dependencies.