resolveInternal restricts usage of embedded compound fields

I did very little debugging, but the problem seems to be that on the Drupal\jsonapi\Context\FieldResolver::resolveInternal() function, the check is only on a dot. If a dot is found, it is assumed to be an external field.

In the Entity Query API, a .entity. part is needed for this external fields, therefore, if a dot is found, it is essentially replaced with .entity.. This means that querying on internal compound fields isn't possible. I think the query syntax should be the same as the Entity API query syntax, meaning that if filtering needs to be done on external fields, the .entity. should be provided in the request.

That would mean an API change for the module, so I haven't created a patch, as I don't know if it really is the best solution.

It seems we could also check if it is an entity_reference field, changing line 73-78 to

      if (
        empty($definitions[$field_name]) ||
        $definitions[$field_name]->getType() != 'entity_reference'
      ) {
        return $external_field_name;
      }

Fixes the issue, but leaves the API intact. Though I think checking if it is not external is the other way around. We should check if the field is internal, and if not, retrieve the external field.