createUnknownError = err =>
    createInvalidLegacyPluginError(
      format('Could not load %s "%s":\n\n %O', pluginType, pluginId, err),
      pluginType,
      pluginId
    )
This should be a unique identifier; either a string (present in `map`),
or a resolvable (via `require.resolve`) module ID/path.
@type {string}