Detect a Drupal 8 site without composer.json for issue #1975220

I could not find the Drupal issue about moving parts of index.php into the core folder.  The check for 'core' and 'composer.json' in the Drupal 7 case is only guarding against false positives, though, so if we simplified it now, and our assumptions were invalidated later, it would not cause huge problems.