<type>processor</type> does not pick up annotation processor

### Affected version

4.0.0-beta-4

### Bug description

While trying out maven 4 with the latest compiler plugin with my private project, I stumbled on something curious with regards to annotation processing.

Scenario:
* run `./mvnw compile` in the root folder of [this project](https://github.com/inaldt/maven4test)
* expected: a class named `MyGeneratedClass` gets generated
* actual: no code gets generated

Note: said class _does_ get generated if one uses the (now deprecated) `AnnotationProcessorPaths` element, as commented out in the pom of the `clientproject` subproject.