Fix broken path resolution when Drupal bootstrapped outside app root

Problem/Motivation

When a DrupalKernel is created with an $app_root it cannot find the service yml files and gives this error:

 [Symfony\Component\DependencyInjection\Exception\InvalidArgumentException]
  The service file "core/core.services.yml" is not valid.

While most of the kernel uses the appRoot to find its files, the service provider discovery part does not.

Proposed resolution

Use the appRoot when loading service ymls.