Move rule support into a new submodule (or remove it)

Problem/Motivation

Phpstan faced error:

Class Drupal\private_message\Plugin\RulesAction\SendPrivateMessage   
extends unknown class Drupal\rules\Core\RulesActionBase.             
💡 Learn more at https://phpstan.org/user-guide/discovering-symbols  

Proposed resolution

Rules module support should be moved into a new submodule (with the appropriate module requirements) or completely removed).