On first-run of a command, or if a command's dependencies change, it's virtualenv should be updated accordingly. Remove all unneeded packages and install/update the ones now needed.

Note that if the current Python version or implementation changes, dependencies should be re-installed.
Raise an error if there's a dependency collision between the installed packages and the global mach venv (see this related bug).

Tweaking this ticket a bit to refer to handling the update efficiently - right now, we blow out venvs and re-create them from scratch, which isn't the fastest.
Instead, we should:

• Verify that venv is structurally sound (unsure how much time should be spent here, maybe more thorough checks should happen in ./mach doctor)
• Update pth file
• Remove unneeded packages, install/upgrade packages