Import a referenced message and return a handle

def _import_message(self, type_name):
        # type: (d.FieldDescriptorProto) -> Text
        """Import a referenced message and return a handle"""
        name = cast(Text, type_name)

        if name[0] == '.' and name[1].isupper() and name[2].islower():
            # Message defined in this file
            return name[1:]

        message_fd = self.descriptors.message_to_fd[name]
        if message_fd.name == self.fd.name:
            # message defined in this package
            split = name.split('.')
            for i, segment in enumerate(split):
                if segment and segment[0].isupper() and segment[1].islower():
                    return ".".join(split[i:])

        # Not in package. Must import
        split = name.split(".")
        for i, segment in enumerate(split):
            if segment and segment[0].isupper() and segment[1].islower():
                assert message_fd.name.endswith('.proto')
                import_name = self._import(message_fd.name[:-6].replace('-', '_') + "_pb2", segment)
                remains = ".".join(split[i + 1:])
                if not remains:
                    return import_name
                raise AssertionError("Don't support nested imports yet")
                # return new_nested_import(import_name, remains)

        raise AssertionError("Could not parse local name " + name)