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

C-family SwiftPM targets now include their headers in the generated Xcode project, with public headers (anything under the public headers directory, recursively) and project/private headers (everything else in the target) matched according to SwiftPM’s own rules. This means headers from package targets such as C libraries with custom module maps, umbrella headers, or nested include directories will appear in the project navigator and build phases as expected. It also applies SwiftPM target exclude paths so excluded headers are not included.