SwiftPM C-family target headers are now included in generated Xcode projects

Generated projects now include the headers from every C-family SwiftPM target (C, C++, ObjC) with the same public/project visibility classification SwiftPM uses. Headers under the package’s public headers directory (default include, walked recursively) become public headers, every other header in the target becomes a project header, and the recognized extensions match SwiftPM’s own rule (h, hh, hpp, h++, hp, hxx, H, ipp, def). Custom module maps, umbrella headers, nested headers, and non-.h extensions that were previously dropped now appear in the generated Xcode project, while .exclude paths on the target are honored.