Overwrite exist method of Twig FilesystemLoader (2.2)

- Implement exist method for the Symfony\Bundle\TwigBundle\Loader\FilesystemLoader to allow bundle syntax for templates

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | fabpot/Twig#1156, #8665
| License       | MIT

The issue with ```The profiler template "SecurityBundle:Collector:security.html.twig" for data collector "security" does not exist.``` also happens in 2.2 so I’m creating this new PR against 2.2.