Add annotation to package-info.java file to workaround MCOMPILER-205

Maven fails incremental builds when there are source files that do not generate class files. One example of this is package-info.java. This (old) issue is tracked at MCOMPILER-205, where the recommended workaround is adding an annotation to the class.

After this, with some other related fixes, I was able to do a proper incremental maven build.