It seems that PsiPolyVariantReferences are not found and returned by
ReferencesSearch.search(). Is this correct? If yes, what's the recommended way to find
such references or is there a better way to implement such "multi-declaration" references?
The better way would indeed be better because currently a PsiPolyVariantReference doesn't
play well together with the Rename refactoring either: The refactoring cannot be triggered
on such a reference, and when triggered on a declaration, the references pointing to it
are not renamed at all :(