[MCOMPILER-412] Support --process-module-path

**[remi forax](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=forax)** opened **[MCOMPILER-412](https://issues.apache.org/jira/browse/MCOMPILER-412?redirect=false)** and commented

> In the modular world, javac will look for annotation processor in the module-path (only if the annotation processor module is required by the module-info, you can use "required static" or uses with the service) and javac will also look to the --processor-module-path with again a nice separation because the source code doesn't see the module inside the module.
>
> Everything works cleanly if there is no module-info.java, if you have a module-info.java, you have two cases
> 1) your annotation processor is itself a module, if you do a requires on it, Maven will put it in the module-path, so it will work (but there is no nice separation provided by --processor-module-path)
> 2) your annotation processor is not a module, you can do a require on it but Maven will not put it on the module-path because it's not a module, so you have to use the \ of the Maven compiler plugin

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**Affects:** 3.8.0, 3.8.1

**Remote Links:**
- [#150 Support --process-module-path
](https://github.com/codehaus-plexus/plexus-compiler/issues/150)