Fix Drupal 9 test failures

Added link to parent issue.

The error Error: Class 'Doctrine\Common\Inflector\Inflector' not found is caused by that Drupal core removed doctrine/common as a dependency for Drupal 9. This library depended on doctrine/inflector, which Feeds uses.

So that means Feeds would need to get a dependency on that library, adding that as a dependency in the composer.json file. It would also be good to add support for Ludwig module, so Feeds remains installable without Composer as well.

This adds a dependency on "doctrine/inflector".

I think that this patch also fixes the \Drupal::$container is not initialized yet. error.

I decided to remove usage of doctrine/inflector now in #3123396: Feeds automatically uses English Plurals for all languages., so Feeds can still be installed on D9 without the need of using of Composer nor Ludwig.

The patch removes the changes to composer.json and removes the no longer needed ludwig.json file.

Temporary added $defaultTheme here to reveal possible more D9 test failures.

Also fixed a deprecation in template_preprocess_feeds_feed() which got revealed in https://www.drupal.org/pift-ci-job/1677572.

This should fix at least some of the new test failures.

assertStringContainsString() is added to D8.8 for forward compatibility in #3126797: [D8 only] Add forwards-compatibility shim for assertString(Not)ContainsString()replacements in phpunit 6&7, so I assume that would not make the D8.8 tests fail here.

The following test failure seems a bit harder to fix, so leaving that one out for now:

Drupal\Tests\feeds\Functional\Update\UpdateNonExistentActionUpdateTest::testUpdateActionsUpdateNonExistent
The update failed with the following message: "Failed: Drupal\Component\Plugin\Exception\PluginNotFoundException: The "node_unpublish_action" plugin does not exist.

Okay, enough tests failures fixed for D9 so far, let's remove $defaultTheme now as that change is causing test failures on D8.8, which is already worked on in #3095636: Add defaultTheme in tests extending BrowserTestBase..

Committed most of #13. Issue remains open because there is still a test failure on a post update (see #12).

The D9 test failure 'Failed: Drupal\Component\Plugin\Exception\PluginNotFoundException: The "node_unpublish_action" plugin does not exist.' happens in EntityProcessorBase::calculateDependencies(). The following piece of code could cause that exception:

$definition = \Drupal::service('plugin.manager.action')->getDefinition($this->getConfiguration('update_non_existent'));
if (isset($definition['provider'])) {
  $this->addDependency('module', $definition['provider']);
}

This made me curious as what happens when you try to perform an import where a cleaning stage occurs, but you have an action configured that no longer exists? Then, bam! Fatal error! That's a bug, so I opened #3138185: Clean stage fails with fatal error when selected action for the 'update_non_existent' setting no longer exists for that.

With #3138185: Clean stage fails with fatal error when selected action for the 'update_non_existent' setting no longer exists committed, all D9 test failures are fixed.