NamespaceLoader currently does not implement create_module or exec_module, which is problematic since load_module is deprecated.

The create_module docstring is the same as _LoaderBasics:

```
def create_module(self, spec):
    """Use default semantics for module creation."""
```

Is it intended to be empty, or is NamespaceLoader unfinished?

If this is intended, how should we create the module instead? If there is other part of the code dealing this, via a fallback in case the loader does not implement this method, wouldn't it make sense to put that in in the loader itself for standalone use?