Package name for .dsl editor is limited to "org.eclipse.gemoc" and dsl.xtext plugins not correctly named

All java package names of the .dsl editor have only `org.eclipse.gemoc` as base instead of `org.eclipse.gemoc.dsl`

This is not precise enough. All these packages must be renamed to use their plugin name as base name.

(note, this was probably due to the original creation using xtext wizard, the refactoring must take care of aslo updating the mwe2 generator file)