Support debugger launch outside venv to debug program in venv

[TheJJ]

debugpy should be able to launch a module/file within a venv, without debugpy being in the venv itself.

This allows having a system installation of debugpy, but use it in venvs.

Currently the venv must be active when launching a debugpy server, but when this venv has

include-system-site-packages = false

which is reasonable and set by e.g. pdm - but debugpy itself can't be launched if it's not installed in the venv then.

-> debugpy should support a venv path setting of a venv to activate right before launching the debugged program.

[rchiodo]

Moving to enhancements for up votes.