isExternalModule = (m, moduleClass) =>
          ['ExternalModule'].includes(moduleClass) &&
          'externalType' in m &&
          m.externalType !== undefined
@param {import('webpack').Module} m
@param {string} moduleClass
@returns {m is import('webpack').ExternalModule} // TODO: this is not
  true anymore, but there's no superclass of all reasonable module
  types