duplicated qName validation

[mchlrch]

(Forked from https://github.com/zazuko/rdf-mapping-dsl/issues/88#issuecomment-650747315)

Validation for duplicated QualifiedNames within the complete project. Validation for all elements that are referenced.

ATM, without validation in place, outgoing references are simply ambiguous if there are multiple targets with the same QName.

This validation should be enabled for e.g.

• DialectGroup
• XmlNamespaceExtension
• LogicalSource
• Vocabulary
• ...

[nnamtug]

Not checking duplication for:

• Referenceable: referenced by Scoping, not globally
• Mapping: never referenced (the name is basically not used at all)

[nnamtug]

Testing:

1. open inner eclipse
2. copy file 'duplication-check.xrm' (and maybe do a project clean)

[mchlrch]

I also added duplication checks for:

• Referenceable, RdfsClass, RdfProperty, Datatype: because I already had issues with duplicated Referenceables in the same LogicalSource
• Mapping: because the name of the mapping is also materialized in the generated output - not having it unique could cause unintended conflicts that way