Apply then flatten nested schema options

def _normalize_nested_options(self):
        """Apply then flatten nested schema options"""
        if self.only is not None:
            # Apply the only option to nested fields.
            self.__apply_nested_option('only', self.only, 'intersection')
            # Remove the child field names from the only option.
            self.only = self.set_class(
                [field.split('.', 1)[0] for field in self.only],
            )
        excludes = set(self.opts.exclude) | set(self.exclude)
        if excludes:
            # Apply the exclude option to nested fields.
            self.__apply_nested_option('exclude', excludes, 'union')
        if self.exclude:
            # Remove the parent field names from the exclude option.
            self.exclude = self.set_class(
                [field for field in self.exclude if '.' not in field],
            )
        if self.opts.exclude:
            # Remove the parent field names from the meta exclude option.
            self.opts.exclude = self.set_class(
                [field for field in self.opts.exclude if '.' not in field],
            )