Generated SPM C-target module-map products no longer break sibling shim modules

SwiftPM C-family targets with an explicit module map now surface all of their headers as project headers rather than Public in the generated framework. Copying them into Framework.framework/Headers previously flipped sibling #if __has_include(<Module/Header.h>) guards (e.g. CNIOBoringSSLShims) and broke consumers like swift-nio-ssl under the MemberImportVisibility upcoming feature. Umbrella-directory targets that expose individual headers like <Module/Header.h> keep their public headers so framework consumers can still import them.