What does this implement/fix? Explain your changes.

There was a recursive dependency in clr module initialization when a public class that implements IEnumerable exists in the global namespace.

The fix is to delay updating clr module dict with contents of .NET namespaces until after our internal modules are loaded because .NET classes might depend on our internal modules to be initialized. In particular, any class implementing IEnumerable needs IterableMixin from collections.py

Does this close any currently open issues?

#1601

Checklist

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• Make sure to include one or more tests for your change