Refactoring: reorg Maven modules for reusability

Let's reorganize our module structure to simplify JJava kernel reusability. The new structure might look like this:

  • jjava-distro
    1. Implements main
    2. Builds the fat jar out of jjava-jupyter, jjava-kernel, jjava-maven
    3. Shades dependencies
    4. Packages the fat jar and jjava-launcher
  • jjava-kernel - most of the Java code from ex. jjava. Classpath magics. Pure library, clean dependencies. No shading or fat jar.
  • jjava-jupyter - a renamed jupyter-jvm-basekernel
  • jjava-maven - Maven magics
  • jjava-launcher - unchanged