Drush 13 compatibility

Problem/Motivation

• Drush 12 now expects all commandfiles in the <module-name>/src/Drush/<Commands|Generators> directory. The Drush subdirectory is a new requirement. (https://www.drush.org/12.x/commands/)
• <module-name>/drush.services.yml is deprecated and will be ignored in Drush 13 (https://www.drush.org/12.x/dependency-injection/)

Those changes will be mandatory for a Drush 13 compatibility.

Steps to reproduce

Proposed resolution

Remaining tasks

• Move Generators directory into a Drush directory
• Adapt namespaces in all Generators classes (ie. namespace Drupal\extra_field\Generators; => namespace Drupal\extra_field\Drush\Generators;
• Inject dependencies through the create() method
• For a Drush 11 backward compatibility: Move generators definitions from drush.services.yml to extra_field.services.yml and adapt their namespace to include the new Drush directory
• Delete drush.services.yml
• Pay attention to #3498759: Update Generators to be compatible with chi-teck/drupal-code-generator 4.x and #3363600: Add support for Drupal Code Generator version 3 (required for drush 12) - some generators might need more changes than just the above

User interface changes

API changes

Data model changes