[SITE-3373] Stop scaffolding the settings.pantheon.php file.

[greg-1-anderson]

This PR allows sites to opt-in to a new method for Pantheon to deliver Drupal integrations to Drupal sites. In the future, we will automate the removal of the scaffolding technique by way of the Composer pre-update script in the Drupal Composer Managed upstream.

Note that the instructions in this PR won't work yet, but the following interim steps probably will (n.b. Drupal 11 / Drupal CMS sites only):

$ terminus connection:set sftp
$ terminus composer require pantheon-systems/drupal-integrations:dev-stop-scaffolding
$ terminus drush ev '\Pantheon\Integrations\Utils::stopScaffolding();'
$ terminus composer update
$ terminus env:commit --message "Stop scaffolding Pantheon's Drupal integrations, and include directly from vendor instead."

This should remove the error messages:

Unable to download modules via the UI: Any packages other than the implicitly allowed packages are not allowed to scaffold files. See the scaffold documentation for more information. pantheon-systems/drupal-integrations

and:

Your site cannot be automatically updated until further action is performed.
Any packages other than the implicitly allowed packages are not allowed to scaffold files. See the scaffold documentation for more information.
pantheon-systems/drupal-integrations

Implementation provided.