Plugin search_api_solr_acquia_multi_subs instance class does not exist

Problem/Motivation

Tried enabling the 1.x-dev version of this module (http://cgit.drupalcode.org/acquia_search_multi_subs/commit/?id=a8e82af) but ran into some errors.

Since I already had Acquia Search enabled, I had a Server and search index automatically setup.

On /admin/config/search/search-api/server/acquia_search_server/edit, when switching to "Acquia Solr Multi Sub" for the Backend of the existing "Acquia Search API Solr server”, there was an Ajax error.

I found out that this error is originating from this line:
$this->backendPlugin = $backend_plugin_manager->createInstance($this->getBackendId()

in Server::getBackend() in Drupal\search_api\Entity\Server.php

I found the following information about the error:

Drupal\Component\Plugin\Exception\PluginException: Plugin (search_api_solr_acquia_multi_subs) instance class "Drupal\acquia_search_multi_subs\Plugin\search_api\backend\SearchApiSolrAcquiaMultiSubsBackend" does not exist. in /var/www/build/html/core/lib/Drupal/Component/Plugin/Factory/DefaultFactory.php on line <i>102</i>

It looks like when trying to create an instance of the Plugin, class_exists("Drupal\acquia_search_multi_subs\Plugin\search_api\backend\SearchApiSolrAcquiaMultiSubsBackend") is returning false in DefaultFactory::getPluginClass() .(Drupal\Component\Plugin\Factory\DefaultFactory.php)

Also related:

I removed the following use lines from SearchApiSolrAcquiaMultiSubsBackend.php since they did not appear to be used:

use Drupal\acquia_connector\Client;
use Drupal\acquia_connector\CryptConnector;
use Symfony\Component\Validator\Constraints\False;

Symfony\Component\Validator\Constraints\False in particular was causing the following fatal error:

PHP Fatal error:  Cannot use Symfony\Component\Validator\Constraints\False as False because 'False' is a special class name in /var/www/build/html/modules/contrib/acquia_search_multi_subs/src/Plugin/search_api/backend/SearchApiSolrAcquiaMultiSubsBackend.php on line 15"

When creating a new Server with the "Acquia Solr Multi Sub" Backend selected, save results in another fatal error:

PHP Fatal error:  Class 'Solarium\Core\Plugin\AbstractPlugin' not found in /var/www/build/html/modules/contrib/acquia_search_multi_subs/src/EventSubscriber/SearchSubscriber.php on line 12"

This problem is different as it appears that the error is coming from attempting to create a new SearchSubscriber() in SearchApiSolrAcquiaMultiSubsBackend::connect()