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
https://github.com/symfony/symfony/issues/11189#issuecomment-46697381
"the PSR-4 rules are applied before the PSR-0 ones"