[2527] Export chained connector ends and the interface keyword

[AxelRICHARD]

Models created before the importer set referencedFeature for owned chains only store the chain as ownedRelatedElement of the ReferenceSubsetting; resolve the referenced feature from there as a fallback. InterfaceUsage must be exported with the interface keyword, which is mandatory in the grammar.

Bug: #2527

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