New php library structure made easier

| Q | A
| ------------- | ---
| Bug fix? | no
| New feature? | yes
| BC breaks? | no
| Deprecations? | no
| Tests pass? | yes
| Fixed tickets | -
| License | MIT
| Doc PR | -

[This article](https://medium.com/@christophewillemsen/stop-making-bundles-think-bundles-deadd27b88c0) from @ikwattro gives some good ideas on how to ease the creation of a PHP package:
- which is not a bundle usable only on a symfony full stack framework
- without requiring to maintain 2 repos (one for the lib and the other for the bundle)

The only drawback is that Symfony requires the DI extension to be on a given location. So I created a new method Bundle#getContainerExtensionClass than can be easily overwritten if you want to move the Extension class in another directory.