Add plugin service descriptor paths to minion classpath

[Vampire]

In my use-case I have a spock test plugin in a separate Gradle source set.
I add the build output to the launch classpath.
The Gradle plugin sets hard includeLaunchClasspath to false.
The build output is added as .../build/classes/java/pitest and .../build/resources/pitest,
where the first contains the classes, the second the service descriptor.

The current logic searches the relevant services and then adds their code source
to the minion classpath.
This works fine if the class and descriptor are in one JAR how it usually is the case.
But for the described case, only the first path is added to the minion classpath
and then the Minion cannot find the configured test plugin.

There are two possible workarounds:

1. pack the class and descriptor in one JAR or directory and add that to the launch classpath
2. add the directories additionally to the classPath or classPathFile argument, so that they are always added to the minion classpath

[Vampire]

I updated this PR to latest master, maybe you have time for it now. :-)