registerI18nLoader = (packageName, localeId, loader) => {
    // register loader by key
    const bundleKey = `${packageName}/${localeId}`;
    loaders.set(bundleKey, loader);
  }
Registers i18n loader function for given package and locale.

@public
@param {string} packageName for which package this loader can fetch data
@param {string} localeId locale that this loader can handle
@param {function} loader async function that will be passed a localeId and should return a JSON object