Individual venv per package

[davecoates]

Currently, using pdm, a single virtualenv is created and used for managing all sub-packages. This isn't ideal as we could run into issues with a dependency being used in a package that isn't listed explicitly in its dependencies.

The problem is there's no real support for the concept of a monorepo like there is in the node world. I think something like this would solve the problem.

For now happy to leave it and see if any solutions appear in PDM itself, otherwise we may revisit

(FWIW it seems to be the same in other tools like Hatch)