Sharing install files between virtualenv instances
The whole point of virutalenv is to isolate and compartmentalize dependencies. What you are describing directly contradicts its use case. You could go into each individual project and modify the environmental variables but that's a hackish solution.
I have 2-3 dozen Python projects on my local hard drive, and each one has its own virtualenv. The problem is that adds up to a lot of space, and there's a lot of duplicated files since most of my projects have similar dependencies.
Is there a way to configure virtualenv or pip to install packages into a common directory, with each package namespaced by the package version and Python version the same way Wheels are?
For example:

~/.cache/pip/common-install/django_celery-3.1.16-py2-none-any/django_celery/
~/.cache/pip/common-install/django_celery-3.1.17-py2-none-any/django_celery/

Then any virtualenv that needs django-celery can just symlink to the version it needs?