Align PyPI package names

[AusafMo]

Summary

• rename Python distributions to cfg-vcs and cfg-impact
• align publish workflow environments, publishing docs, and install commands with those names
• update optional-dependency error messages to point at the published distributions

Review notes

• I did not keep the original tiny message-only diff as-is because it referenced cfg-vcs without updating pyproject metadata or publishing docs. This PR keeps the package names coherent against remote main.

Tests

• .venv/bin/python -m pytest -q
• .venv/bin/python -m build
• .venv/bin/python -m twine check dist/*
• (cd plugins/cfg_impact && ../../.venv/bin/python -m build && ../../.venv/bin/python -m twine check dist/*)
• clean wheel install smoke with cfg_vcs wheel + cfg_impact wheel