Stop SwiftPM C-target headers from breaking sibling shim modules

For SwiftPM C-family targets brought in as custom-/header-style modules (consumed via import Module), all discovered headers are now surfaced as project headers instead of Public. Marking them Public previously copied them into the generated framework’s Headers/ directory, which flipped sibling shim targets’ #if __has_include(<Module/Header.h>) guards and broke the CLI’s own swift-nio-ssl build. Directory-style (umbrella) targets imported as <Module/Header.h> keep their public headers Public so they still land in the framework bundle.