Scratch dictionary being prematurely emptied in codemod

Running a codemod command like:

`python3 -m libcst.tool codemod --module="x" ensure_import_present.EnsureImportPresentCommand PATH`

results in:
```
self.context.scratch["module"],
KeyError: 'module'
```

I see the scratch dict is being [replaced](https://github.com/Instagram/LibCST/blob/47e5ea15e1c92358f3c7da6af11ac46fb80ba78d/libcst/codemod/_cli.py#L251) with an empty one before anything happens with the context object.

The codemod works when I hard code the scratch dict

Is this a bug or am I missing something here?