Add a pre_install_action to stop add-on from installing into empty directory

[japerry]

As part of install, this add-on requires Drupal to already be downloaded to the destination project folder.

Therefore this add-on should either:

• Add a pre_install_actions hook that errors out if there is no drupal project in the folder
  -or-
• Automatically download Drupal (aka drupal/recommended-project) before continuing.

I realize for development, users would/should have their project configured, but if this add-on is used for demos or streamlining environment setup, less steps would be good.