Entity query doesn't allow relationship specifier for base entity fields

Problem/Motivation

#2424791: Entity query hardcodes entity_reference and entity specifier allows adding relationship specifier to the EFQ. Sample usage can be found at https://www.drupal.org/node/2835034. EFQ doesn't accommodate relationship specifier for basefields.

Proposed resolution

See #7

Remaining tasks

Finalize the approach. Add tests.

User interface changes

None.

API changes

It replicates the API added in #2424791: Entity query hardcodes entity_reference and entity specifier for basefields.

Data model changes

None

Original report by @heddn

I'm using DER with the rng module. Rng has a base DER field for events. I've implemented https://www.drupal.org/node/2835034, but I always get a query exception in Tables::ensureEntityTable(), saying that 'event' property isn't found. It isn't. It is called 'event__target_id' and 'event__target_type'. Am I doing the query incorrectly?

    /** @var \Drupal\commerce_product\Entity\ProductVariation $variation */
    $variation = reset($variations);
    $entityTypeId = $variation->getProduct()->getEntityTypeId();
    $idKey = $this->entityTypeManager->getDefinition($entityTypeId)
      ->getKey('id');
    $query = $this->registrationStorage->getQuery()
      ->condition("event.entity:$entityTypeId.$idKey", $variation->getProductId());
    $count = $query->count()->execute();