WIP: Issue #3215025: Unable to restore from backup (No hook functions were found for provision-deploy-backup)

This is an attempt to reinstate the ability to restore an environment from backup. Currently, this fails on the UI and command line with "No hook functions were found for provision-deploy-backup)."

Marking the merge request "WIP" for now, as it seems to me that the include statements at the bottom of provision.drush.inc should not be necessary--I wasn't able to get the associated inc files to autoload or their functions to run otherwise, however. Hopefully someone else can point me in the right direction here.

Thank you!