Action migration source moved to system module

The Action migrate source plugin, action migrations and tests have moved from the Action module to the System module which is where action entities are managed. The Action module only provides a UI.

For site builders it means that they no longer need to enable the Action module to migrate actions.

Any custom code using the Action source plugin needs to update the namespace and make sure any tests also references the moved source plugin.

Before
\Drupal\system\Plugin\migrate\action\Action
After
\Drupal\system\Plugin\migrate\source\Action

Also moved are;

The migrations

action_settings.yml
d6_action.yml
d7_action.yml

The tests

 core/modules/{action => system}/src/Plugin/migrate/source/Action.php
 core/modules/{action => system}/tests/src/Kernel/Migrate/d6/MigrateActionConfigsTest.php
 core/modules/{action => system}/tests/src/Kernel/Migrate/d6/MigrateActionsTest.php
 core/modules/{action => system}/tests/src/Kernel/Migrate/d7/MigrateActionConfigsTest.php
 core/modules/{action => system}/tests/src/Kernel/Migrate/d7/MigrateActionsTest.php
 core/modules/{action => system}/tests/src/Kernel/Plugin/migrate/source/ActionTest.php