Add functionality for getHierarchy() to get different plugin types

With last patch hierarchy facets don't show their own configuration forms. Attached one that achieve this. Also trying to fix failing tests (I don't have experience with tests coverage and not sure if test will be fixed).

Sorry wrong patch (forgot to add the changes on \Drupal\facets\Form\FacetForm). This should work.

Missing wrong ['settings'] key on form.

Patch fails a bunch of tests. Those should probably be fixed. I think this is much needed though, because we only have one hierarchy plugin we're not 100% sure that the capabilities for supporting other plugins.

Using this patch, I am able to create a new FacetsHierarchy plugin and use that to create nested hierarchical facet lists in my current project. As far as I can tell, I didn't encounter any bug that seems related to this patch.

The main thing that I missed is a code reference to the facet that a hierarchy plugin is attached to, so that I can access the facet configuration from any of the methods in HierarchyInterface. I solved that by adding the facet id to the hierarchy settings via buildConfigurationForm, so that I can load the respective facet in any of the interface methods using an additional method that I added to my plugin class:

/**
   * Get the facet that this processor is attached to.
   *
   * @return \Drupal\facets\FacetInterface
   */
  protected function getFacet() {
    $facet_id = $this->getConfiguration()['source_facet'];
    return \Drupal::entityTypeManager()->getStorage('facets_facet')->load($facet_id);
  }

I'm using custom hierarchy on top of hierarchical integer lists, patch is working perfectly

Rerolled patch on 8.x-1.x-dev and update some annotations.

We are probably going to need to update the tests so they use the new schema's. Overall this is looking great and it already has an upgrade path, will commmit + tag a new release when this passes tests.

Rerolling the patch to the latest release.

We'd remove one TODO but add three! This new patch simply removes all these TODOs and does not pretends that we support plugin configuration (the previous patches do not support it neither). It might be added properly in follow-up.

I'll provide more patch to fix tests, but upload this one as the base.

Would this fix tests?

This should fix schema error.

And this one should fix the dependency on the taxonomy module.

This one fixes testHierarchySettings().

Sorry that I don't provide interdiff. These are to fix tests, so it'd better to review only the last patch.

Fix the normalized config tests.

This should fix the remaining fails.

Same as #28 but this one removes Facet::loadHierarchies(), this is useless because getHierarchies() and loadHierarchies() always return the same thing.

Fix coding standards.

wonderful!

Added reroll of patch #30 for 8.x-2.x to fix the error for the duplicate update hook facets_update_8007() after updating to 8.x-2.x

With the help of this patch I created a custom FacetsHierarchy plugin, which now works with 8.x-2.x.

Reroll after #3255596: Fix coding standards was committed.

Attached patch fixes code style errors, but is otherwise unchanged.

I tested this patch manually and added a new plugin. It is selectable in the setting. But neither the new one nor the old plugin work. Both result in
Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" plugin does not exist. Valid plugin IDs for Drupal\facets\Hierarchy\HierarchyPluginManager are: taxonomy, new_hierarchy_plugin in Drupal\Core\Plugin\DefaultPluginManager->doGetDefinition() (line 53 of core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php).

I think the update hook misses the default configuration.

In an update hook we don't have the entity API in place (unlike in a post update), so preSave is not called.