Updating process requirement so it can be included in other projects with symfony 4 and updating vendor code.

Issue #2542236 by colan: Excluded directories that shouldn't be in site backups, and added alter hook.

Improve error message for "Platform not found" state: Show the directory and mention all of the problems that could exist.

Issue #2960237 by Jon Pugh: Sites on remote servers fail on Platforms that use git or composer, with Drupal in a subfolder

Revert "Generate service.yml file to set cookie domains, and thus fix link construction on subdir sites."

This reverts commit b441a70e.