Notebook imports and completions from package dependencies needs new unit tests

[swernli]

Currently all our tests for completions in notebooks only use items defined in the standard library or within the notebook itself. With the new support for external projects in notebooks, we need some virtual file system based tests for completions in notebooks.

[minestarks]

Partially addressed in #1947 (unit tests for completions from package dependencies), but not for notebooks yet.