Set gradle annotationProcessorPath to compileClassPath

[glefloch]

This branch set the annotationProcessorPath of the gradle compile task to point to the compileClasspath.

By default annotation processors in the compileClasspath are not executed. It requires dependency to be declared with the annotationProcessor scope.

close #15104

[gastaldi]

That makes sense

[aloubyansky]

Thanks @glefloch A comment about that would be useful to have in the code. So we don't forget why we did it and what will fail in case it gets removed.

[glefloch]

@aloubyansky you are right, I added a comment :)

[gastaldi]

Question: wouldn't this override existing annotationProcessorPaths defined for the compile task?

[glefloch]

I just tested it. If a user sets it in its build.gradle, it won't be overridden by the plugin.