Support for Terms having Multiple Parents

Similar to #498182: Support for a term with multiple parents →

When looking at <a href="https://api.drupal.org/api/drupal/core%21modules%21taxonomy%21src%21TermStorage.php/function/TermStorage%3A%3AloadTree/8.8.x">TermStorage::loadTree</a>, we see:

         if (isset($this->treeParents[$vid][$load_entities ? $term
            ->id() : $term->tid])) {

            // Clone the term so that the depth attribute remains correct
            // in the event of multiple parents.
            $term = clone $term;
          }

Therefore, we need to filter out duplicates.

I created a new method to preprocess the result of TermStorage::loadTree (including removing the duplicates):

  /**
   * Preprocesses the vocabulary taxonomy tree
   */
  protected function getVocabularyTreeArray($vocabulary_tree, $image_field, $vocabulary) {
     ...
  }

I don't like the additional two parameters: $image_field and $vocabulary.

Ideally, we create another method to process the config for us:

  /**
   * Get volcabulary name and image field
   */
  protected function getVocabularyConfig() {
    $vocabulary_config = $this->configuration['vocabulary'];
    $vocabulary_config_split = explode('|', $vocabulary_config);

    return [
      'vocabulary' => $vocabulary_config_split[0] ?? null,
      'image_field' => $vocabulary_config_split[1] ?? null,
    ]
  }

Or even better, we preprocess the form options when submitting to form, to create the additional keys: $this->configuration['vocabulary_name'] and $this->configuration['image_field'].

Okay, remove the two additional parameters by just duplicating the processing config approach:

  /**
   * Preprocesses the vocabulary taxonomy tree
   */
  protected function getVocabularyTreeArray($vocabulary_tree) {
    // TODO: How can we avoid this duplicated work?
    $vocabulary_config = $this->configuration['vocabulary'];
    $vocabulary_config = explode('|', $vocabulary_config);
    $vocabulary = isset($vocabulary_config[0]) ? $vocabulary_config[0] : NULL;
    $image_field = isset($vocabulary_config[1]) ? $vocabulary_config[1] : NULL;

    ...
  }