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
note that the advantage only appears in dev though. In prod, you should dump an optimized autoloader with composer, which will then never reach the PSR-4 or the PSR-0 logic normally but only the classmap