Function package_dir_path 

fn package_dir_path(root: &Path, full_name: &str) -> Option<PathBuf>

Where the layout would keep a package’s published versions: the full name’s components (namespace, then name) become path components under root. Only builds the path — existence is the caller’s question. None for names the layout can’t hold (empty or path-like components), rather than letting them escape the root.