function makeRootPlugin(ModulesInRootPlugin, name, root) {
  if (typeof root === 'string') {
    return new ModulesInRootPlugin(name, root);
  }
  if (isArray(root)) {
    return function () {
      root.forEach(function (root) {
        this.apply(new ModulesInRootPlugin(name, root));
      }, this);
    };
  }
  return function () {};
}
webpack 1 defaults: https://webpack.github.io/docs/configuration.html#resolve-packagemains
@type {string[]}