function makeWebpackExternalsConfig(moduleNames) {
  return moduleNames.reduce((externals, moduleName) => Object.assign(externals, {
    [moduleName]: `commonjs ${moduleName}`,
  }), {});
}
Make a webpack 'externals' object from the provided CommonJS module names.
@param  {Array<String>} moduleNames The list of module names
@return {Object}                    A webpack 'externals' config object