bug #29833 [DebugClassLoader] expose proxyfied findFile() method (fancyweb)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #29833).

Discussion
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[DebugClassLoader] expose proxyfied findFile() method

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

As bad as it is, some third party libraries expect that at least one autoload function will be the Composer one and have behaviors that relies on the public `findFile` method.

When the `DebugClassLoader` wraps the Composer `ClassLoader`, the function `findFile` is currently lost. So it becomes impossible to use the `DebugClassLoader` with these libraries.

This is for example the case in Drupal 😢 (cf 83bc30ac40/lib/Drupal/Component/ClassFinder/ClassFinder.php).

Fixing these bad implementations in third party libraries can take forever as things move way slower than in Symfony. This is why I think supporting this case directly in Symfony is better. It's easy and will make the `DebugClassLoader` compatible with more cases.

What could be done to go further in this direction would be to proxify any method implementend by wrapped class loaders.

Commits
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4f690a31c6 [DebugClassLoader] Readd findFile() method

src/Symfony/Component/Debug/DebugClassLoader.php

@@ -124,6 +124,11 @@ class DebugClassLoader
         }
     }
 
+    public function findFile($class): ?string
+    {
+        return $this->isFinder ? $this->classLoader[0]->findFile($class) ?: null : null;
+    }
+
     /**
      * Loads the given class or interface.
      *