Very nice PR, @slbmeh. I'll be sure to read through it closer soon. Your work on getting Wordpress to be composer.json-compliant is great, and definitely something that I see the DrupalInstaller benefiting from.

This PR mimics the effect that the Drupal command line utility has, and wouldn't require the ```extra``` notes in most composer.json files. I understand having a composer.json in a child path and not within the root is somewhat backwards from a best practice Composer project, but that is unfortunately how Drupal is designed. There are plans to change that within the next couple years though, however, for Drupal 9. We could adapt this solution quite easily to match accordingly by then.