Warn user if other modules were added or updated before install

According to the docs in package_manager.api.php

 * - \Drupal\package_manager\Event\PostRequireEvent
 *   Dispatched after one or more Composer packages have been added to the
 *   stage directory. This event may be dispatched multiple times during the
 *   stage life cycle, and receives a list of the packages which were required
 *   into the stage directory. (Note that this is a list of packages which
 *   were specifically *asked for*, not the full list of packages and
 *   dependencies that was actually installed.)

Doesn't this mean PostRequireEvent can't tell us everything that will be changed?

In talking to Ted, we can check (1) what was asked for by us vs. (2) what was actually installed.
- We need our own validator--will deal with presenting the confirmation page, probably which subscribes to StatusCheck.

We would write a validator like \Drupal\automatic_updates\Validator\StagedProjectsValidator (this shows how we can see what changed)

In the rare case that it might remove a dependency, we might want it to be an actual ERROR, because we'll want them to uninstall the module first.

Should we check if database updates are needed after install? Worth a look in a separate issue. (They already have a StageDbUpdateValidator)

Allow proceed only on _warnings_ coming back.

Should ALSO subscribe to PreApply, so we can see if if it's about to remove a module that is currently enabled. Force user to uninstall.

We fire StatusCheck. PreApply is a PM one.

WIP MR in progress. It implements the PreApplyEventSubscriber but nothing with StatusCheck yet.