Automatically select interpreter when VIRTUAL_ENV is set

[brettcannon]

When a virtual environment is activated the VIRTUAL_ENV environment variable is set to the location of the virtual environment. We could pick up on this and use it as the implicit value of python.pythonPath when that setting has not already been set.

[alexwhittemore]

You might have just gone above my head a little; I don't really understand the difference. But it does appear to be the case that it's not quite accurate, at least not in the cases that a user would recognize. Namely, when I put in place my goofy hack from the latest comment at #2333, it all "Just Works" the way I expect/think it should. It seems to me like re-activating a detected virtualenv in ~/.bash_profile wouldn't work so effectively if what you're saying is true. Does that make any sense?

[brettcannon]

I get it, but I think we might be talking about different things in terms of how/where code is being executed.

[karrtikr]

Closing as dup of #18178.