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
What's the disadvantage of using PSR-4? I don't except anyone who start a new symfony project to create PEAR-like classes, do you?