Include SwiftPM C-family target headers in generated projects

Generated projects now include headers from C-family SwiftPM targets, aligned with how SwiftPM classifies them. Headers under the public headers directory (typically include, recursively) are marked as public headers, while all other recognized headers in the target are treated as project (private) headers. This covers nested headers, non-.h extensions such as .hpp and .hxx, and targets with custom module maps or umbrella headers. The discovery also respects SwiftPM target exclude paths, so excluded headers are not added. Previously, only top-level include/*.h headers were included in a narrow case, and many C-family targets appeared with missing headers in the generated Xcode project.