minor #17145 Do not use HttpKernel Extension when not needed (Tobion)

This PR was merged into the 2.3 branch.

Discussion
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Do not use HttpKernel Extension when not needed

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

HttpKernel Extension only adds `addClassesToCompile`. So the class hierarchy should be slim if it's not used.

Commits
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4978e19 Do not use HttpKernel Extension when not needed

Author: fabpot

Tests/Fixtures/ExtensionLoadedBundle/DependencyInjection/ExtensionLoadedExtension.php

@@ -12,7 +12,7 @@
 namespace Symfony\Component\HttpKernel\Tests\Fixtures\ExtensionLoadedBundle\DependencyInjection;
 
 use Symfony\Component\DependencyInjection\ContainerBuilder;
-use Symfony\Component\HttpKernel\DependencyInjection\Extension;
+use Symfony\Component\DependencyInjection\Extension\Extension;
 
 class ExtensionLoadedExtension extends Extension
 {

Tests/Fixtures/ExtensionPresentBundle/DependencyInjection/ExtensionPresentExtension.php

@@ -12,7 +12,7 @@
 namespace Symfony\Component\HttpKernel\Tests\Fixtures\ExtensionPresentBundle\DependencyInjection;
 
 use Symfony\Component\DependencyInjection\ContainerBuilder;
-use Symfony\Component\HttpKernel\DependencyInjection\Extension;
+use Symfony\Component\DependencyInjection\Extension\Extension;
 
 class ExtensionPresentExtension extends Extension
 {