Swap to using isinstance of mixins per review.

Co-authored-by: Patrick Lehmann <[email protected]>

pyVHDLModel/__init__.py

@@ -1016,11 +1016,13 @@ def _ImportObjects(package: Package) -> None:
 			for referencedLibrary in package._referencedPackages.values():
 				for referencedPackage in referencedLibrary.values():
 					for declaredItem in referencedPackage._declaredItems:
-						if hasattr(declaredItem, "_normalizedIdentifiers"):
+						if isinstance(declaredItem, MultipleNamedEntityMixin):
 							for normalizedIdentifier in declaredItem._normalizedIdentifiers:
 								package._namespace._elements[normalizedIdentifier] = declaredItem
-						else:
+						elif isinstance(declaredItem, NamedEntityMixin):
 							package._namespace._elements[declaredItem._normalizedIdentifier] = declaredItem
+						else:
+							raise VHDLModelException(f"Unexpected declared item.")
 
 		for libraryName in ("std", "ieee"):
 			for package in self.GetLibrary(libraryName).IterateDesignUnits(filter=DesignUnitKind.Package):  # type: Package