Cannot create Mapper for other created files

[smotastic]

If a Mapper needs to be created for a Class which is also generated by source_gen, the mapper generator will throw an error that the source / target class is not a ClassElement.

The problem is, that my generator might, or might not run before the generator which will create the dependend class.

So the smartstruct_generator should always run after all class files which are potentially needed, are created.

See https://pub.dev/packages/build_config#adjusting-builder-ordering

[smotastic]

See #25