This PR was merged into the 1.x branch.

Discussion
----------

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
-------

2c174e4 Use class_exists instead of require

This PR was merged into the 2.7 branch.

Discussion
----------

[TwigBridge] Fix namespaced classes

| Q             | A
| ------------- | ---
| Branch?       | 2.7 <!-- see comment below -->
| Bug fix?      | yes
| New feature?  | no <!-- don't forget updating src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget updating UPGRADE-*.md files -->
| Tests pass?   | yes
| Fixed tickets | #23072 <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | N/A

Commits
-------

a1cdc2d [TwigBridge] Fix namespaced classes