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.

Start by tracing how mypy reports missing imports and how it determines the running interpreter from sys.executable. Compare that prefix with the VIRTUAL_ENV value, and decide whether the warning should be limited to missing-import cases and recommend --python-executable; done means the behavior and scope are covered by tests.