def get_bin_dir(self, build_module: str) -> str:
        """Return a path to the binaries dir for a build module dir.
           Create sub-tree of missing dirs as needed, and return full path
           to innermost directory.
        """
        bin_dir = os.path.join(self.conf.get_bin_path(), build_module)
        if not os.path.isdir(bin_dir):
            # exist_ok=True in case of concurrent creation of the same dir
            os.makedirs(bin_dir, exist_ok=True)
        return bin_dir