Move config import-related events out of ConfigEvents, into a new ConfigImporterEvents

Problem/Motivation

In the wake of #3405800: Config collections do not trigger configuration events consistently, we realized that the config events are all lumped into one class (\Drupal\Core\Config\ConfigEvents), which is a bit overloaded. In particular, these events:

• \Drupal\Core\Config\ConfigEvents::IMPORT_VALIDATE
• \Drupal\Core\Config\ConfigEvents::IMPORT
• \Drupal\Core\Config\ConfigEvents::IMPORT_MISSING_CONTENT
• \Drupal\Core\Config\ConfigEvents::STORAGE_TRANSFORM_IMPORT
• \Drupal\Core\Config\ConfigEvents::STORAGE_TRANSFORM_EXPORT

All of these operate on complete storages, not on individual config objects, regardless of which collection they're in. They should probably be moved to their own class to make it clearer what they're for -- Drupal\Core\Config\ConfigImportEvents or ConfigStorageEvents. Name TBD.

Proposed resolution

Move the constants I just listed into a new class, and deprecate the old ones.

Remaining tasks

Well, just moving code around and adding some deprecation comments, really.

API changes

Yes, this will deprecate some existing event constants.

Release notes snippet

TBD but we'll likely need something here.