Allow reuse in Export DSL

[rmitin]

Export declarations of one DSL might be also used by another DSL if that one, for example, creates derived models of the first DSL. I.e. many languages could create derived Java types and export them.

• Suggest syntax to reuse export declarations and fingerprint definitions (forcing the developer to create new *.export files with reusable definitions is fine).
• Implement
• Provide examples using hello world test DSL