lib/classes/PluginManager.js/PluginManager/loadCorePlugins
class PluginManager:  loadCorePlugins() {
    const pluginsDirectoryPath = path.join(__dirname, '../plugins');

    const corePlugins = this.serverless.utils
      .readFileSync(path.join(pluginsDirectoryPath, 'Plugins.json')).plugins
      .map((corePluginPath) => path.join(pluginsDirectoryPath, corePluginPath));

    this.loadPlugins(corePlugins);
  }