Use PSR-4 everywhere instead of PSR-0

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11189
| License       | MIT
Well, both of these standards are valid IMO (even though I also prefer the PSR-4). And your change is valid.

But here, on the root, we have a PSR-0 structure (the src is in the `src` directory, which have a `Symfony` subdir, which have a `Component`, `Bridge` and `Bundle`subdirectory, ... etc), when in the bundles / components, we have a PSR-4 structure (the sources are on the same level as the composer.json).

I'm just not sure if there is a gain of performances (it depends if the psr-4 stuff is loaded before or after the psr-0 stuff)