#331 remove remaining runtime dependencies from processor to mapstruct core

... and add checkstyle rule to verify that for the future.

As an alternative to using the checkstyle rule, we could also pull out the prism generation to its own module with a "provided" dependency to ```mapstruct``` and then use ```mapstruct``` only with scope test in ```processor```. 
When shading away freemarker, we would create a fat jar anyway and thus we could merge the prism classes into that jar as well.