*FileGenerator* renamed to *SourceGenerator*

this include the name of the package were all the classes used to generate source of classes from the class annoted with @Mapper
this renaming is motivated by the futur use of these classes to generate source into String and not only into File
moved packages writer and imports under package sourcegenerator

these packages are only used by classes inside the sourcegenerator package
renamed artifact core to core-parent

in process, bump version number to 0.2.0-SNAPSHOT to avoid side effects
fixed recursive dependency of artifact model on itself
change motivated by the need to have a jar artifact with an artifact id core
moved packages sourcegenerator and validator from module annotation-processor to module core

moving these packages out of annotation-processor module will allow us to have source generating and DASourceClass validating code in the classpath without the annotation-processor (thus triggering code generation which do not necessarily want)