Notebook imports and completions from package dependencies needs new unit tests

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.

Start by locating the existing notebook completion tests and the new external-project support. Add virtual-file-system-based unit tests covering imports and completions from package dependencies, alongside the current standard-library and notebook-local cases. Done means the dependency completion scenarios are covered by passing tests.

Written by the indexing model from the issue text.