Allow use without activation of virtualenv

[ssbarnea]

Allow users to call ansible-lint with a full path even when it is installed inside an inactive virtualenv.

This change also includes a fail-safe test that prevents future regressions.

Fixes: #1507
Closes: #1866 (duplicate)

[webknjaz]

I just saw #1866 and I like it better because of a more accurate approach and no import-time patches.

[ssbarnea]

@webknjaz I had a talk with @cidrblock and I will attempt to remove the code running at import from linter before doing this change. If it works we may be able to ditch that anti-pattern.

[ssbarnea]

I am very aware about the other change which is altering a huge amount of code, likely 10x more than this one and that is a big deal.

I do not want to modify all subprocess calls to inject full paths in order to address this issue. That may come back to us very soon and it might conflict with other use cases and we could easily introduce bugs when calling other processes and forgetting to add the magic sanitization call to them. At least the PATH adulteration at start is clear and ensures that whatever we call will be using the same rules.

I also sorted the import issues and avoided run of code as import.