code: function configureExternals(config) {
    if (!externals)
      return
    config.externals = {}
    for (let {globalIdentifier, importIdentifier} of externals)
      config.externals[importIdentifier] = globalIdentifier
  }
docs: Exclude the target module from the bundle and set the global variable that
  identifies it. This is usually used in conjuction with the HTML plugin to
  insert CDN links at the “head” tag.