Don't try to automatically register Composer Stager services

phenaproxima → credited catch → .

Crediting @catch for suggesting this solution.

phenaproxima → credited tedbow → .

I've gone over this with @tedbow, and tests are passing on the current minor, previous minor, and previous major. This is needed by Drupal CMS and so I will be committing it at 4 PM, North America eastern time, unless there's some strong reason not to do so.

Merged into 3.1.x.

Automatically closed - issue fixed for 2 weeks with no activity.