SwifterPM registry symlinks no longer break generated Xcode projects

Local tuist install against SwifterPM now keeps package roots as real workspace directories under .build/registry/downloads, with top-level cache entries symlinked into them, so Xcode sees the same package root shape SwiftPM exposes and builds registry packages (including those with transitive C/C++ headers, e.g. Firebase Firestore) reliably while the efficient in-place SwifterPM cache is still used. CI still uses copies via the existing automatic mode.