Fix for GWT XML files not being resolved from compileSourceArtifacts

The readModule() method doesn't include the compileSourceArtifacts and so is unable to resolve the GWT XML files regardless of whether the sources have been correctly added to the compile classpath.

I've overridden the getClassPath() method and moved the property "String[] compileSourceArtifacts" into the parent class (I had to change this from private to protected).

This has fixed the issue that we have in that our published API jars don't contain the GWT XML files but the sources obviously do.