redo python venv machinery. Allow poetry

[mxcl]

Refs #211

[mxcl]

A significant difference to the previous way we stubbed is that Python is no longer a direct dependency of the package, but is instead loaded inside the shim.

This means that packages can use entirely different python versions to those that may be set in the user’s environments. Which specifically works around conflicts like: user wants python 3.12 and poetry, a tool they are using locally, requires 3.11.

However, this means any package that moves to this shim needs RESTOCKING, since the dep has been removed from the yaml.

This is one of the reasons I am considering moving to a system where each package records its deps alongside the package at build time.