It should be faster (#35) and work on Windows (#11)

Haven't tried it yet, but read through the changes and it looks fantastic. Great job.

@greg-1-anderson Why is expected that the index.php is not present after the initial composer install? Both packages (the scaffold and drupal core) are in the composer.json. I can't spot the difference to the previous implementation.

I just ran phpunit locally, and the tests passed. I'm going to have to scratch my head a bit about why the failure is happening on Travis; the cause is not immediately apparent to me.

The only difference with the previous implementation was that the robo task was exec'ed, whereas now the files are fetched directly from the composer plugin process. However, I cannot reproduce this locally, so it's hard for me to imaging what it is about the Travis system that causes the test to fail there.

Does a local phpunit run pass for you? I presume that it probably does.

#40 failed after composer update. I change the test to match the new behaviour. It makes more sense anyway. Thanks @greg-1-anderson for your help.

LGTM. Some minor changes to drupal-composer/drupal-project will be needed once this is merged.

@greg-1-anderson Yes the version constraint. What else?

I added a PR, drupal-composer/drupal-project#155 to demonstrate. I think it's about right, but have not tried it.