feature #24257 [HttpKernel][DI] Enable Kernel to implement CompilerPassInterface (nicolas-grekas)

This PR was squashed before being merged into the 3.4 branch (closes #24257).

Discussion
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[HttpKernel][DI] Enable Kernel to implement CompilerPassInterface

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

In the same spirit as #13761 that allowed DI exts to be also compiler passes, and as #23812 that allowed the kernel to listen to events, in our new bundle-less world, should we allow the kernel to register itself as a compiler pass? That would make some scenario possible (like having a `TestKernel` that turns some services public.)

Commits
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6973a1a [HttpKernel][DI] Enable Kernel to implement CompilerPassInterface

Author: nicolas-grekas

CHANGELOG.md

@@ -4,6 +4,7 @@ CHANGELOG
 3.4.0
 -----
 
+ * moved the `ExtensionCompilerPass` to before-optimization passes with priority -1000
  * deprecated "public-by-default" definitions and aliases, the new default will be "private" in 4.0
  * added `EnvVarProcessorInterface` and corresponding "container.env_var_processor" tag for processing env vars
  * added support for ignore-on-uninitialized references

Compiler/PassConfig.php

@@ -45,10 +45,10 @@ public function __construct()
                 new ResolveInstanceofConditionalsPass(),
                 new RegisterEnvVarProcessorsPass(),
             ),
+            -1000 => array(new ExtensionCompilerPass()),
         );
 
         $this->optimizationPasses = array(array(
-            new ExtensionCompilerPass(),
             new ResolveChildDefinitionsPass(),
             new ServiceLocatorTagPass(),
             new DecoratorServicePass(),