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.
this signature looks weird, becayse it assumes that the extension name will be based on the bundle name without the ``Bundle`` suffix