…ause the name is already in use (ogizanagi)

This PR was merged into the 3.3-dev branch.

Discussion
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[DI] Fix Cannot declare class ...\DefinitionDecorator, because the name is already in use

| Q             | A
| ------------- | ---
| Branch?       | 3.3
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #21369
| License       | MIT
| Doc PR        | N/A

The `return` trick doesn't seem to work, and php is still trying to declare the `DefinitionDecorator` class, which causes the "Cannot declare class ...\DefinitionDecorator, because the name is already in use" error because of the `class_alias` previously declared in `ChildDefinition.php`.

This never happens as soon as the `ChildDefinition` class is used first, as the alias will take hand, but their are some situations, like in some unit test cases it can happen apparently, because `DefinitionDecorator` is used first.

Commits
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530849e [DI] Fix Cannot declare class ...\DefinitionDecorator, because the name is already in use

…sDecisionManager BC layer (ogizanagi)

This PR was merged into the 3.3-dev branch.

Discussion
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[Security] Fix TraceableAccessDecisionManager / DebugAccessDecisionManager BC layer

| Q             | A
| ------------- | ---
| Branch?       | 3.3
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #22657 (comment)
| License       | MIT
| Doc PR        | N/A

Same as #22657 for the renaming of `DebugAccessDecisionManager` into `TraceableAccessDecisionManager`. Indeed, I think we also require to redeclare the old `DebugAccessDecisionManager` for composer `classmap-authoritative` autoloading strategy & classmap dumper.

AppVeyor failures unrelated.

Edit: Re-thinking about it, it's probably not very common to dump the classmap and use the `classmap-authoritative` strategy when using those classes... That's to say: in debug mode/dev env. So it may be the reason why the class wasn't redeclared on contrary of `DefinitionDecorator`.

Commits
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5b123b9 [Security] Fix TraceableAccessDecisionManager / DebugAccessDecisionManager BC layer

This PR was merged into the 1.x branch.

Discussion
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Use class_exists instead of require

I think this is required for symfony/symfony#23073 to pass. It's probably very similar to symfony/symfony#22657.

A simple reproducer:

```php
<?php

use Twig\Node\Node;

require_once __DIR__ . '/vendor/autoload.php';

new Node();
new \Twig_Node();
```

[Without this patch](https://travis-ci.org/symfony/symfony/jobs/239712676#L2100):

```php
PHP Fatal error:  Cannot declare class Twig_Node, because the name is already in use in vendor/twig/twig/lib/Twig/Node.php on line 20
```

With, everything works fine, whatever the two calls order.

Commits
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2c174e4 Use class_exists instead of require