[PATCH 5 of 8] import-checker: skip site-packages for virtualenv

[timeless]

> pygments was being treated as a stdlib...

Yuya Nishihara <yuya at tcha.org> wrote:
> If I understand it, pygments should be treated as a stdlib. stdlib_modules is
> used to sort out system-installed modules from our modules.

so, isort has 3 categories: stdlib, other, local

If what we're trying to capture is "modules that are part of
mercurial" vs "modules that are not part of mercurial", we should
rename the concept to "non_mercurial_modules" or "external_modules".

Personally, I think I favor isort's approach and would suggest we
probably want to sort into three categories instead of two categories.

But we can do that later.