bug #20749 [FrameworkBundle] Smarter default for framework.annotations (ogizanagi)

This PR was merged into the 3.2 branch.

Discussion
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[FrameworkBundle] Smarter default for framework.annotations

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | yesish (could be considered as a minor BC break)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

`framework.annotations` default should be true only if `doctrine/annotations` is installed.

Indeed, in symfony/symfony#20097, the dependency on `doctrine/annotations` was removed from the framework bundle.
Thus, an application can break (not talking from one actually relying on annotations) as soon as it uses the framework bundle without the `framework.annotations` key explicitly set to `false` (I had the case in a fixture application in the testsuite of a package).

Commits
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e38be09 [FrameworkBundle] framework.annotations default should be true only if doctrine/annotations is installed

Author: fabpot

DependencyInjection/Configuration.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Bundle\FrameworkBundle\DependencyInjection;
 
+use Doctrine\Common\Annotations\Annotation;
 use Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition;
 use Symfony\Component\Config\Definition\Builder\TreeBuilder;
 use Symfony\Component\Config\Definition\ConfigurationInterface;
@@ -602,7 +603,7 @@ private function addAnnotationsSection(ArrayNodeDefinition $rootNode)
             ->children()
                 ->arrayNode('annotations')
                     ->info('annotation configuration')
-                    ->canBeDisabled()
+                    ->{class_exists(Annotation::class) ? 'canBeDisabled' : 'canBeEnabled'}()
                     ->children()
                         ->scalarNode('cache')->defaultValue('php_array')->end()
                         ->scalarNode('file_cache_dir')->defaultValue('%kernel.cache_dir%/annotations')->end()