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
I'm not sure the root composer.json should change to PSR-4. It is valid for the components' and bundles', because it replaces the deprecated composer's `target-dir` option, but on the root, as there is no such option enabled...