Add support for BatchOperations dependencies

swirt → credited Mile23 → .

Crediting Mile23 for the suggestion of adding this.

There is some logic now present to determine if a batch operation has been run before. I am not clear if this need would be to only run A if B. and C were just run .. or if they were ever previously run.

It is already possible through using the 'preBatchMethod($sandbox)' or 'postBatchMethod($sandbox)' to call another BatchOperation directly so a dependency could actually be performed.

Maybe this just needs documentation as an example BatchOperation.

Example:
If A needed B and C to run first, then In the batch operation for A add the optional method

public function preBatchMethod($sandbox) {
  $B = \Drupal::classResolver('\Drupal\YOUR_MODULE\cbo_scripts\B');
  $B->run($sandbox, 'hook_update');

  $C = \Drupal::classResolver('\Drupal\YOUR_MODULE\cbo_scripts\C');
  $C->run($sandbox, 'hook_update');

  return "I ran B and C.  They will each have their own logs that you should see.";
}