Function submodule_modules_git_dir 

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

Filesystem path to the separate git directory for a submodule at submodule_worktree_rel (path relative to the superproject work tree), under super_git_dir.

Git nests additional modules/ segments for each path component (e.g. path a/b → <super>/modules/a/modules/b), not a single modules/a/b directory.