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

Just to unblock the issue on drupal.org, I pushed a change that tests for composer.json in either location--at the Drupal root, or inside the core directory. @weitzman is correct that the check for Drupal 7 (composer.json does not exist)  should also be updated.

The question is, what is the best signature to use to identify that a 'core' directory  is a Drupal 8 core directory?  Perhaps core.services.yml is good -- probably better than composer.json.  Due to my haste this morning, I didn't scroll all the way to the right to notice the filename change from composer.json here.

If we agree on the best check to make, I'll go ahead and fix up both the Drupal 8 and Drupal 7 tests.