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

Includes and replaces #11190 + psr-4 support for ClassNotFoundFatalErrorHandler (not sure if this is considered as a new feature, it's kinda trivial)
Out of curiosity : Why so many `dirname`s, instead of `../`s ?