fix nebari invocation through python

[pmeier]

What does this implement/fix?

In its current state, python -m nebari ... does not work, since the imports fail. The whole point of the __main__ file is to enable that. This PR fixes the imports and adds a test to avoid the same issue in the future.

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

[iameskild]

LGTM! Thanks @pmeier :)

[iameskild]

/bot run tests