fix: ignore conflicting symlinks at site-packages root on venv creation

[mattem]

Handle conflicting symlinks when creating the venv directory so that the last link wins if the existing link can be removed.
Also bails early with better error message when conflicts that aren't at the root, or caused by namespace packages are found.

Closes #335

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: no
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

Handle conflicting symlinks at venv root during venv creation.

Test plan

• New test cases added
• Manual testing; please provide instructions so we can reproduce:

Force a conflict and observe error message.

[vors]

🙌