Remove all old-style service aliases

Problem/Motivation

We have many old service aliases for Composer Stager's core services, plus a lot of our internal event subscribers. Things like package_manager.beginner, package_manager.stager, and package_manager.committer, and so forth.

The thing is, we now generally refer to services by their class or interface name, since Drupal core now supports that, and it also makes decoration (a pattern we use heavily) more straightforward. We should not keep these old aliases; we should standardize on class/interface name.

Proposed resolution

Change all references to those services:

package_manager.beginner --> \PhpTuf\ComposerStager\API\Core\BeginnerInterface::class
package_manager.stager --> \PhpTuf\ComposerStager\API\Core\StagerInterface::class
package_manager.committer --> \PhpTuf\ComposerStager\API\Core\CommitterInterface

...and so on, until there are no more arbitrary service aliases left in our code base.