Update common/setup.py and documentation around virtualenv

### Context

After jumping back into this project after a while I ran into an issue where my local build of common was removed. This lead me down a rabbit hole of trying to debug the build process where setuptools was missing during common build.

The topic is vaguely touched on in the current README but I think we can do better in two regards. The documentation should call out the types of scripts that required the virtualenv to be activated. Also, we should explicitly test of the existence of the virtual env in common.py or check if setuptools is installed and display a message if it is not available that explains how to resolve the issue.

### Solution or next step

- Update documentation to show how virtualenv is utilized
- Update `common/setup.py` to warn users of missing dependency and how to resolve
- (optionally) check if virtualenv is activated by checking `sys.prefix`