Fix default main actor isolation mapping for SwiftPM packages

Tuist now maps SwiftPM’s default main actor isolation to the correct Xcode build setting MainActor for generated external package projects. Previously, the value was emitted as main-actor, which Xcode ignored, so packages relying on defaultIsolation(MainActor.self) were compiled as if no isolation was requested. This preserves the correct Swift concurrency behavior when integrating those dependencies.