Function register_fetched_source 

pub fn register_fetched_source(read_dir: &Path, name: &str)

Register a builtins.fetch* result directory with the store-path NAME CppNix would give its copied-to-store tree ("source" for fetchGit/fetchTree/fetchTarball). Idempotent per read_dir. Does not affect any string value — only the name a later copy-to-store coercion of this exact path assigns to its store path.