async function loadSource(pathFolders, ...fileNameParts) {
  const paths = lookupFile(pathFolders, ...fileNameParts).reverse();
  return (await Promise.all(paths.map(name => fs.readFile(name, 'utf8')))).join('\n');
}
@params {!Array<string>} pathFolders
@param {!Array<string>} fileNameParts
@return {!Promise<string>}