Get a tuple of (name, fs) that the given path would map to.

        Arguments:
            path (str): A path on the filesystem.
            mode (str): An `io.open` mode.

def which(self, path, mode="r"):
        # type: (Text, Text) -> Tuple[Optional[Text], Optional[FS]]
        """Get a tuple of (name, fs) that the given path would map to.

        Arguments:
            path (str): A path on the filesystem.
            mode (str): An `io.open` mode.

        """
        if check_writable(mode):
            return self._write_fs_name, self.write_fs
        for name, fs in self.iterate_fs():
            if fs.exists(path):
                return name, fs
        return None, None