Function fetch_git_source 

pub fn fetch_git_source(
    git_src: &GitSource,
    cache_base: &Path,
    module_name: &str,
    printer: &Printer,
) -> Result<PathBuf>

Clone or fetch a git source to the cache, returning the local path.

If the repo is already cached, fetches updates. Otherwise, clones. Checks out the specified tag/ref if provided.