Add directory to store built-in plugins to `module_dir` parameter.
        Default directory to store plugins is `BLACKBIRD_INSTALL_DIR/plugins`.
        :rtype: None
        :return: None

def add_default_module_dir(self):
        """
        Add directory to store built-in plugins to `module_dir` parameter.
        Default directory to store plugins is `BLACKBIRD_INSTALL_DIR/plugins`.
        :rtype: None
        :return: None
        """
        default_module_dir = os.path.join(
            os.path.abspath(os.path.curdir),
            'plugins'
        )
        module_dir_params = {
            'module_dir': [default_module_dir]
        }

        if 'module_dir' in self.config['global']:
            module_dir_params['module_dir'].append(
                self.config['global']['module_dir']
            )
        self.config['global'].update(
            module_dir_params
        )