SwifterPM registry downloads and static xcframework module maps no longer break Xcode builds

Generated Xcode projects now build cleanly when using SwifterPM with registry packages that pull in C and C++ headers, and when consuming static Objective-C xcframeworks that ship their own module map (such as CLibsql). SwifterPM registry downloads under .build/registry/downloads are now materialized as real directories whose top-level entries symlink into the SwifterPM cache, preserving the workspace shape Xcode expects during dependency scanning. For static xcframeworks whose slice already provides a module.modulemap, the selected headers and module map are now exposed through a derived headers directory, so the module is defined exactly once and clang dependency scanning no longer sees duplicates.