Use proper source_base_path configuration for D8.2.x

Now that #2695297: Refactor EntityFile and use process plugins instead has been committed, with Drupal 8.2.x and up the source_base_path value needs to be set under source configuration rather than destination configuration - without this change, file migration will not work. This change will be made in the 8.x-3.x branch, which supports Drupal 8.2.x and up, and the 8.x-2.x branch will only support Drupal 8.1.x.