Fix mocking for modules with folders on windows

[KELiON]

Summary

Fixing #4014: mocks for packages with folders (like lodash/fp/sum) on Windows.

Questions

1. what is the better way to write tests for that?
2. Is this change enough? Doesn't it break something else on windows? There are not a lot for tests

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[cpojer]

This works for me, do you mind running yarn lint -- --fix and pushing to this branch again?

[KELiON]

@cpojer sure, done

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