Add support for --relocatable

[yeswalrus]

--relocatable was an experimental feature in older versions of the virtualenv script. We have a few use cases where it would be helpful to be able to package up a venv and copy it to a different machine, and really the only part of the activate script that we have to change is the VIRTUAL_ENV environment variable.
Would it be possible to revive this old flag & useful feature, and just have it replace the activate script's VIRTUAL_ENV='<absolute path>' with VIRTUAL_ENV='$(realpath $(dirname $0)/..)' or something like that?

[nikaro]

Likely fixed by #5515