Generated Xcode projects now include SwiftPM C-family target headers

SwiftPM C-family targets (custom module maps, umbrella headers, nested headers, non-.h extensions such as .hpp and .hxx) now contribute their public and project headers to the generated Xcode target with the same public/project visibility distinction SwiftPM applies. Downstream code can #include these headers from the generated project, unblocking packages like OpenSwiftUI from generating with Tuist.