feature symfony#26341 Autoconfigure service locator tag (apfelbox)

This PR was squashed before being merged into the 4.1-dev branch (closes symfony#26341).

Discussion
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Autoconfigure service locator tag

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#26187
| License       | MIT
| Doc PR        | —

We will now also autoconfigure the (required) tag `container.service_locator` for all `ServiceLocator`s.

I couldn't find any integration tests for the whole list of autoconfigured core services, so I didn't add any new tests.

Commits
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c4b9b42 Autoconfigure service locator tag

Author: nicolas-grekas

src/Symfony/Bundle/FrameworkBundle/CHANGELOG.md

@@ -16,6 +16,7 @@ CHANGELOG
  * Deprecated `bundle:controller:action` syntax to reference controllers. Use `serviceOrFqcn::method` instead where `serviceOrFqcn`
    is either the service ID or the FQCN of the controller.
  * Deprecated `Symfony\Bundle\FrameworkBundle\Controller\ControllerNameParser`
+ * The `container.service_locator` tag of `ServiceLocator`s is now autoconfigured.
 
 4.0.0
 -----

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

@@ -42,6 +42,7 @@
 use Symfony\Component\DependencyInjection\ParameterBag\ContainerBagInterface;
 use Symfony\Component\DependencyInjection\ParameterBag\ParameterBagInterface;
 use Symfony\Component\DependencyInjection\Reference;
+use Symfony\Component\DependencyInjection\ServiceLocator;
 use Symfony\Component\DependencyInjection\ServiceSubscriberInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
@@ -288,6 +289,8 @@ public function load(array $configs, ContainerBuilder $container)
             ->addTag('config_cache.resource_checker');
         $container->registerForAutoconfiguration(EnvVarProcessorInterface::class)
             ->addTag('container.env_var_processor');
+        $container->registerForAutoconfiguration(ServiceLocator::class)
+            ->addTag('container.service_locator');
         $container->registerForAutoconfiguration(ServiceSubscriberInterface::class)
             ->addTag('container.service_subscriber');
         $container->registerForAutoconfiguration(ArgumentValueResolverInterface::class)