Allow TemplateManager to fallback to SF TemplateNameParser & TemplateLocator

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | *no*
| Fixed tickets | fabpot/Twig#1156, #8665
| License       | MIT

The ```exist``` method of the ```Symfony\Bundle\TwigBundle\Loader\FilesystemLoader``` (inherited from ```Twig_Loader_Filesystem```) doesn’t fallback to Symfony’s TemplateNameParser & TemplateLocator and therefore fails on template names following the Bundle syntax like ```SecurityBundle:Collector:security```.

This is option 1 how to fix this. However, I prefer option 2..see #8266.