Implement proper dependency injection where possible

Why do you need this:

  public function getService() {
    return $this->service ?: \Drupal::service('service_id');
  }

If you've set the service in create() then it's there. At least that's how every other plugin I've worked with so far works. Is SearchAPI doing something different with the plugin manager?

I seem to remember reading somewhere that the reason entity classes can't have service properties is that the entity object it sometimes serialized.

I don't really like the setter-injection and prefer constructor-only injection myself but overall this patch looks great, let's go ahead and commit this.

This is a bit of a tangent, but this got me thinking:

I don't really like the setter-injection and prefer constructor-only injection myself

While developing some process plug-ins myself, I noticed this difference from core. I've been using constructor-only for so long, and it generally keeps things fairly simple, but sometimes you end up with a really unwieldy mess whenever a sub-class has to inject an additional dependency, like in this example from a plug-in unrelated to Search API:

  public function __construct(
    array $configuration, string $pluginId, array $pluginDefinition,
    ModuleHandlerInterface $moduleHandler,
    LanguageManagerInterface $languageManager,
    RendererInterface $renderer,
    SerializationInterface $yamlSerialization,
    TranslationInterface $stringTranslation,
    CacheBackendInterface $htmlCacheService,
    MimeTypeGuesserInterface $mimeGuesser
  ) {
    parent::__construct(
      $configuration, $pluginId, $pluginDefinition,
      $moduleHandler,
      $languageManager,
      $renderer,
      $yamlSerialization,
      $stringTranslation,
      $htmlCacheService
    );

    // Save dependencies.
    $this->mimeGuesser = $mimeGuesser;
  }

...which feels a bit comical just to inject one dependency. In this case, setter-injection seems like it would be better as it doesn't require the sub-class to be updated if the ancestor changes its __construct() parameters. What are your thoughts on this?