/**
 * Legacy CommonJS main resolution:
 * 1. let M = pkg_url + (json main field)
 * 2. TRY(M, M.js, M.json, M.node)
 * 3. TRY(M/index.js, M/index.json, M/index.node)
 * 4. TRY(pkg_url/index.js, pkg_url/index.json, pkg_url/index.node)
 * 5. NOT_FOUND
 * @param {URL} packageJSONUrl
 * @param {PackageConfig} packageConfig
 * @param {string | URL | undefined} base
 * @returns {URL}
 */
function legacyMainResolve(packageJSONUrl, packageConfig, base) {
  const packageJsonUrlString = packageJSONUrl.href;

  if (typeof packageJsonUrlString !== 'string') {
    throw new ERR_INVALID_ARG_TYPE('packageJSONUrl', ['URL'], packageJSONUrl);
  }

  const baseStringified = isURL(base) ? base.href : base;

  const resolvedOption = FSLegacyMainResolve(packageJsonUrlString, packageConfig.main, baseStringified);

  const baseUrl = resolvedOption <= legacyMainResolveExtensionsIndexes.kResolvedByMainIndexNode ? `./${packageConfig.main}` : '';
  const resolvedUrl = new URL(baseUrl + legacyMainResolveExtensions[resolvedOption], packageJSONUrl);

+ if (resolvedOption !== legacyMainResolveExtensionsIndexes.kResolvedByMain)
  emitLegacyIndexDeprecation(resolvedUrl, packageJSONUrl, base, packageConfig.main);
  return resolvedUrl;
}