Add support for Drupal Code Generator version 3 (required for drush 12)

Problem/Motivation

First RCs of drush 12 have appeared and they require DCG v3. This module requires
DrupalCodeGenerator\Command\Plugin\PluginGenerator to be present so it can't be installed alongside drush 12.

Steps to reproduce

Setup a drupal installation with this module and drush 12 then run a drush command like drush cr to see a fatal error:
Error: Class "DrupalCodeGenerator\Command\Plugin\PluginGenerator" not found

Proposed resolution

Find a way to ignore the v2 service definition (drush.services.yml) and create a new class for v3 support, hopefully autodiscovered just like drush commands.

Remaining tasks

Code a solution

User interface changes

None

API changes

None

Data model changes

None