Return the src URI from the dst filepath.

        This does not rely on the destination filepath actually
        existing on the local filesystem, just on pattern matching.
        Return source URI on success, None on failure.

def dst_to_src(self, dst_file):
        """Return the src URI from the dst filepath.

        This does not rely on the destination filepath actually
        existing on the local filesystem, just on pattern matching.
        Return source URI on success, None on failure.
        """
        m = re.match(self.dst_path + "/(.*)$", dst_file)
        if (m is None):
            return(None)
        rel_path = m.group(1)
        return(self.src_uri + '/' + rel_path)