Extensions Mapper caches full path which breaks with dynamic containers

CRM_Extension_Mapper caches the full path to extensions which with dynamic containers (such as with Pantheon) means that the cached path might not match the active container's path. This causes a fatal error.

The only solution seems to be to force CRM_Extension_Mapper to fetch the extensions paths from the database, or prevent it from caching the full path. Currently with the CiviCRM Starterkit we are forcing it to fetch the list from the database each time (https://www.drupal.org/node/2347897).

Relates to cloud-native#6 (moved)