search_exclude indexing doesn't update totals with 8.8.x core, no search results

Problem/Motivation

As of #3075695: Move search_index() and related functions to a service Drupal core changed the way search indexing works.
This module was never updated to match the new system.
This means that anything you try to index with core 8.8.x silently "works", but the {search_totals} table is never updated.
If you start with a clean search index on an 8.8.x core site, you'll never get any search results displayed, since the query for search results JOINs against {search_totals}, which is empty.

Calling this "critical" since the fundamental functionality of the module is now broken.

Proposed resolution

Update src/Plugin/Search/SearchExcludeNodeSearch.php to use core's new API.

In particular:

SearchExcludeNodeSearch::updateIndex() currently does this:

    $node_storage = $this->entityManager->getStorage('node');
    foreach ($node_storage->loadMultiple($nids) as $node) {
      $this->indexNode($node);
    }

Core's NodeSearch::updateIndex() now looks more like this:

    $node_storage = $this->entityTypeManager->getStorage('node');
    $words = [];
    try {
      foreach ($node_storage->loadMultiple($nids) as $node) {
        $words += $this->indexNode($node);
      }
    }
    finally {
      $this->searchIndex->updateWordWeights($words);
    }

Remaining tasks

1. Update the search_exclude plugin (see #2), ideally in such a way that it still works with both 8.7.x core and 8.8.x (see #3).
2. Consider adding automated tests to this module so we would have noticed this sooner.
3. Review.
4. RTBC.
5. Commit.

User interface changes

None.

API changes

TBD.

Data model changes

None.

Release notes snippet

TBD.