[Suggestion][Makefile] Create a unified directory for virtualenv

[Hritik14]

A VIRTUALENV_DIR or a hard coded venv directly could be used to store virtualenv files rather than dumping them in the project directory.

This would avoid lines like https://github.com/nexB/scancode.io/blob/1285a9a5cbc8b582bb94ae877241b011856789c2/Makefile#L84-L85

Also, we could ditch the MANAGE variable which hard codes the virtualenv activation directory. This could be of use in pre-activated virtualenvs. We could instead use ${ACTIVATE} /.manage.py A similar approach is at https://github.com/nexB/vulnerablecode/blob/3343dc4f24562dad05e500ea3b155d487a1b6797/Makefile#L98-L99