Add support for JPMS modules

[mtf90]

This PR adds support for JPMS modules introduced in Java 9. It adds module-info.java descriptors for (Maven-) modules that have (Java-) module-aware dependencies and Automatic-Module-Names for modules that don't. The build process now uses two compilation phases to

1. verify and compile the contents of the module-info.java files and to
2. recompile everything with Java 8 target level to maintain compatibility.

The project documentation explicitly enforces modules in order to use their aggregating properties to better structure the generated documentation. The respective source.jars still target Java 8.