Default Content now uses Importer and Exporter services, instead of combined DefaultContentManager; service names changed as well

To isolate concerns of Import and Export, default_content now provides 2 services default_content.import and default_content.export to replace the previous combined service default_content.manager.

To inject dependency on the new services, please use the service names and/or Drupal\default_content\ImportInterface and Drupal\default_content\ExportInterface as appropriate. The new services maintain feature parity, down to method names, implying minimal downstream changes.

Before

$manager = \Drupal::service('default_content.manager');
$manager->importContent('mymodule');
$entity_id = 3;
$manager->exportContent('my_entity_type', $entity_id);

After

$importer = \Drupal::service('default_content.import');
$exporter = \Drupal::service('default_content.export');
$importer->importContent('mymodule');
$entity_id = 3;
$exporter->exportContent('my_entity_type_id', $entity_id);