Fix plugin descriptor extraction

The latest maven-plugin-plugin uses annotation-based extraction by default. Because the whole Maven
plugins ASM dependencies are stupidly broken on a old ASM 3.x version that shokes on post Java7
bytecode, we need to stick to doclets.

This change provides explicit configuration for the 'java' extractor.

It took me some time to figure out what the extractor name could be given how broken the
documentation is at times.