Investigate dropping virtual envs from the python-based plugins

[tigarmo]

What needs to get done

Instead of creating a venv in the part's install dir and pip-installing the packages, pip-install them directly into the Python installation (which must always exist).

Why it needs to get done

The Python and Poetry plugins are currently broken in 24.04 because of the new usrmerge handling in noble. Might as well take this opportunity to address this "liability". This also lets us drop our custom sitecustomize, and bypasses the need to think about symlinks.

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3657.

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