Language Server reports false positives in Native Notebook

[DonJayamanne]

• Install VSCode Insiders
• Install Jupyter Extension
• Install Pylance
• Select Python 3.6 as workspace interpreter
• Select a different interrpeter as Kernel in Notebook (e.g. Python 3.7 Conda)
• Install pandas into the kernel interpreter
• Write code in cell and import pandas

Pylance will report errors about missing packages (pandas is reported as not being installed).
This is a false positive, the user has it installed in the python environment used by the kernel.

The problem is pylance doesn't know that the document is using a different interpreter.

This will eventually get even more complicated:

• Assume we import a python module from the notebook
• I now go to deffinition & land in the python file
• Now what Interpreter should the langauge server use when checking pacakges/imports for this file?
  Basically it depends on the context, but does the LS know this?

[rchiodo]

Dupe of #6333