Fix support for folders in python.pythonPath setting

Environment data

• VS Code version: 1.54.3
• Extension version (available under the Extensions sidebar): v2021.3.680753044
• OS and version: Windows 10.0.19041
• Python version (& distribution if applicable, e.g. Anaconda): 3.8.6
• Type of virtual environment used (N/A | venv | virtualenv | conda | …): venv (via poetry)
• Value of the python.languageServer setting: Pylance

Expected behaviour

For "python.pythonPath": "${workspaceFolder}/.venv" workspace setting, extension is expected to find the virtualenv in the given directory.

Actual behaviour

The extension is unable to find the virtual env with the latest update (rolling back to the previous version works fine).

Steps to reproduce:

1. Create a virtualenv in the workspace possibly using poetry, and point to the workspace with ${workspaceFolder}/{workspace_name}

IMPORTANT NOTE: ".venv\\Scripts\\python.exe" works fine, but "${workspaceFolder}\\.venv", "${workspaceFolder}/.venv" does not. All of them works in the previous version.

Possibly related to #15781 but I do not have the error described in that issue in the output below.

Logs

Output for Python in the Output panel (View→Output, change the drop-down the upper-right of the Output panel to Python)

User belongs to experiment group 'pythonaa'
User belongs to experiment group 'pythonJoinMailingListVar2'
User belongs to experiment group 'pythonJediLSP'
User belongs to experiment group 'pythonSendEntireLineToREPL'
User belongs to experiment group 'pythonNotDisplayLinterPrompt'
User belongs to experiment group 'pythonDiscoveryModuleWithoutWatcher'
User belongs to experiment group 'pythonFindInterpreter'
User belongs to experiment group 'ShowExtensionSurveyPrompt - enabled'
User belongs to experiment group 'CollectLSRequestTiming - control'
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
Starting Pylance language server.
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
> c:\Users\erena\OneDrive\Desktop\SNLab\soccerbars/.venv c:\Users\erena\.vscode\extensions\ms-python.python-2021.3.680753044\pythonFiles\pyvsc-run-isolated.py -c "import sys;print(sys.executable)"
Error 2021-03-25 12:24:27: Diagnostic Code: InvalidPythonInterpreterDiagnostic, Message: No Python interpreter is selected. You need to select a Python interpreter to enable features such as IntelliSense, linting, and debugging.
Diagnostic Code: InvalidPythonInterpreterDiagnostic, Message: No Python interpreter is selected. You need to select a Python interpreter to enable features such as IntelliSense, linting, and debugging.

[meakbiyik]

@karrtikr thanks a lot! Just to note my use case: I use poetry in all my projects with with the poetry.toml:

[virtualenvs]
create = true
in-project = true

looking like this, and "python.pythonPath": "${workspaceFolder}/.venv". So, when someone clones the repository and just runs poetry install, vscode uses the created environment and everything runs perfectly, with zero OS dependence. If vscode decides that we have to explicitly write the binary, though, then the OS independence would be lost since what vscode is automatically doing right now would have to be done manually by the user (and less meta-shenanigans the better especially for people who are not familiar with vscode and/or python). If you can take this use case into consideration that would be really amazing (this might be the single most important setting that has to be shared between the developers/users of a python package for consistency).

[karrtikr]

FYI Insiders build should now have the fix.