Fix main actor isolation mapping for external SwiftPM dependencies

SwiftPM packages that opt into default main actor isolation via .defaultIsolation(MainActor.self) are now correctly mapped to Xcode’s SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor build setting. Previously, Tuist emitted main-actor, which Xcode ignored, so external packages compiled without the expected concurrency isolation. Nonisolated targets continue to map to nonisolated.