Function submodule_modules_git_dir 

pub fn submodule_modules_git_dir(
    super_git_dir: &Path,
    submodule_relpath: &str,
) -> PathBuf

Filesystem path to the separate git directory for a submodule under super_git_dir.

Matches Git submodule_name_to_gitdir with extensions.submodulePathConfig off: append modules/<name> by splitting only on / and \\ (components like .. are kept; no normalization).