Provide a symfony/flex recipe

[pounard]

Right now, bundle class is src/Bridge/Symfony/DbToolsBundle.php but we kept src/DbToolsBundle.php extending it for backward compatibility.

Symfony Flex searches for <your PSR-4>[Bundle]/<your PSR-4>Bundle and automatically append the bundle if it finds it, but once we'll remove the BC compat layer, it won't work anymore.

We have until 3.0 to find an elegant solution, a recipe which is not in the community repository because it's not enabled per default (and nobody enables it in real life).