Deprecate config_overlay_install_tasks_alter() and config_overlay_install_config_overlay()

Ha, noticed that we cannot use ModuleHandlerInterface::loadInclude() as Config Overlay is not installed at that moment (inherently), so we need to manually require the file. Updated the issue summary...

Hmm... yeah I think all of that stuff can be deprecated in favor of pointing people to 🐛 Dispatch config transformation event during site install from configuration Active if they encounter any installation issues.

Tentatively renaming the issue...

Really wavering back and forth on this one a bit...

Tried what @bircher mentioned in the core issue in #5: To add the config overlay config transformer via settings.php. That's not super intuitive, though you need this whole things in your services.yml:

services:
  # See config_overlay.services.yml
  config_overlay.extension_storage_factory:
    class: Drupal\config_overlay\Config\ExtensionStorageFactory
    arguments: ['@extension.list.profile']
  config_overlay.config_subscriber.transform_early:
    class: Drupal\config_overlay\EventSubscriber\ConfigTransformEarlySubscriber
    arguments:
      - '@extension.list.profile'
      - '%install_profile%'
      - '@config_overlay.extension_storage_factory'
      - '@config.storage'
    tags:
      - { name: event_subscriber }

And then, because Config Overlay is not in the autoloader by default, the following in your project's composer.json:

    "autoload": {
        "classmap": [
            "web/modules/contrib/config_overlay/src/Config/ExtensionOptionalStorage.php",
            "web/modules/contrib/config_overlay/src/Config/ExtensionStorageFactory.php",
            "web/modules/contrib/config_overlay/src/Config/ReadOnlyUnionStorage.php",
            "web/modules/contrib/config_overlay/src/EventSubscriber/ConfigTransformEarlySubscriber.php",
            "web/modules/contrib/config_overlay/src/EventSubscriber/ConfigTransformSubscriberBase.php"
        ]
    },

Not sure if there's a way we can tidy that up a bit, but not sure I want to actually recommend people doing so, as it seems like a lot of work...