Detect if shell is in activated virtual environment, but mypy is not installed in it.

Feature

Detect if $VIRTUAL_ENV is defined when running mypy, potentially only if we report missing imports.

Pitch

Right now, if mypy is installed globally, and a user activates a virtual environment, they could be confused about missing import errors (e.g. https://github.com/python/mypy/issues/7237) where the packages are installed in the virtual environment but mypy can't see them. In such a scenario, we'd be able to detect that $VIRTUAL_ENV is set, and we could report a warning if that path doesn't match the prefix of sys.executable.

One question about this is should we report this in general or only if we report a missing import.

I think there probably are use cases where people don't care about running mypy globally, so my initial inclination would be to only report this and suggest --python-executable if an import is missing.

Beginne damit, nachzuverfolgen, wie mypy fehlende Importe meldet und wie es den laufenden Interpreter anhand von sys.executable bestimmt. Vergleiche dieses Präfix mit dem Wert von VIRTUAL_ENV und entscheide, ob die Warnung auf Fälle fehlender Importe beschränkt werden sollte, und empfehle --python-executable; abgeschlossen ist die Aufgabe, wenn Verhalten und Geltungsbereich durch Tests abgedeckt sind.