Implement auto-imports (remove imports from UML)

[lecoqlibre]

Currently, imported elements are specified from the UML model.

For instance, the class Address imports the IAddress interface. But since Address implements IAddress we could assume that the interface must be imported.

Instead of specifying all imported elements in the UML, we could write an Acceleo query to compute elements that need to be imported at code generation time. This would ease the development of the UML model and avoid mistakes.

Note: this will break existing codegen as the import logic should be rewritten.