lib/classes/PluginManager.js/PluginManager/loadServicePlugins
class PluginManager:  loadServicePlugins(servicePlugs) {
    const servicePlugins = (typeof servicePlugs !== 'undefined' ? servicePlugs : []);

    // we want to load plugins installed locally in the service
    if (this.serverless && this.serverless.config && this.serverless.config.servicePath) {
      module.paths.unshift(path.join(this.serverless.config.servicePath, 'node_modules'));
    }

    this.loadPlugins(servicePlugins);

    // restore module paths
    if (this.serverless && this.serverless.config && this.serverless.config.servicePath) {
      module.paths.shift();
    }
  }