feature #36739 [TwigBundle] Deprecate the public "twig" service to private (fancyweb)

This PR was merged into the 5.2-dev branch.

Discussion
----------

[TwigBundle] Deprecate the public "twig" service to private

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| Deprecations? | yes
| Tickets       | -
| License       | MIT
| Doc PR        | -

I think the `twig` service don't need to be public anymore - we never need to access it directly in Symfony's code.

Commits
-------

f64cbada89 [TwigBundle] Deprecate the public "twig" service to private

Author: fabpot

Tests/Functional/BundlePathsTest.php

@@ -40,7 +40,7 @@ public function testBundlePublicDir()
     public function testBundleTwigTemplatesDir()
     {
         static::bootKernel(['test_case' => 'BundlePaths']);
-        $twig = static::$container->get('twig');
+        $twig = static::$container->get('twig.alias');
         $bundlesMetadata = static::$container->getParameter('kernel.bundles_metadata');
 
         $this->assertSame([$bundlesMetadata['LegacyBundle']['path'].'/Resources/views'], $twig->getLoader()->getPaths('Legacy'));

Tests/Functional/app/BundlePaths/config.yml

@@ -8,3 +8,8 @@ framework:
 
 twig:
     strict_variables: '%kernel.debug%'
+
+services:
+    twig.alias:
+        alias: twig
+        public: true