minor #30023 [Config] fix compat with wrapping autoloaders (nicolas-grekas)

This PR was merged into the 3.4 branch.

Discussion
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[Config] fix compat with wrapping autoloaders

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

Registering this private method as autoloader works, until one wants to decorate autoloaders (like our DebugClassLoader does.)

Commits
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695d84be4c [Config] fix compat with wrapping autoloaders

Author: nicolas-grekas

Resource/ClassExistenceResource.php

@@ -118,8 +118,10 @@ public function unserialize($serialized)
 
     /**
      * @throws \ReflectionException When $class is not found and is required
+     *
+     * @internal
      */
-    private static function throwOnRequiredClass($class)
+    public static function throwOnRequiredClass($class)
     {
         if (self::$autoloadedClass === $class) {
             return;