SwifterPM registry package roots no longer break Xcode dependency scanning

SwifterPM registry downloads now keep each registry package root as a real workspace-local directory under .build/registry/downloads, with its top-level entries symlinked into SwifterPM’s cache, rather than a single symlink for the whole root. Generated Xcode projects now see the same root shape that SwiftPM exposes, so transitive C/C++ headers resolve correctly under dependency scanning while local restores still avoid a recursive copy. Continuous integration still uses copies in automatic mode.