fetchBackerData = async function(force) {
  logger.info("fetchBackerData");
  const backers = yaml.safeLoad(await readFile(backersPath, "utf8"));
  for (const backer of backers.items) {
    if (backer.githubUser)
      await cacheAvatarImageForGithubUser(backer.githubUser, force);
  }
}
Fetch backer data
@param {Array} packageNames
@param {Boolean} force