Add message on the reroute settings form if Drupal Symfony Mailer is enabled

Added a messages with instructions for setting up the module with Symfony Mailer.

Hi @abyss,
thanks for your contribution!

1) We need to add a few really simple tests:
a) for the main module: none of those messages are added on the settings page.
b) for submodule (which has Drupal Symfony as its dependency):
- the correct message is added on the settings page if Symfony Mailer is enabled and submodule is not.
- no messages are added on the settings page if Symfony Mailer and submodule are installed.

2) We need to add hook_requirements to the main module.
So, if we have Symfony Mailer is enabled and submodule is not, we will see an notice at Status report page.

3) This is a bit weird to have multiple addMessage in a row. Please replace the logic with:

if ($this->moduleHandler->moduleExists('symfony_mailer')) {
      if ($this->moduleHandler->moduleExists('reroute_email_symfony_mailer')) {
         $this->messenger->addMessage(WHOLE MESSAGE #1);
      }
      else {
         $this->messenger->addMessage(WHOLE MESSAGE #2);
      }
  }

Committed to 2.3.x dev branch. It will be a part of the next 2.3 series releases.
Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.