fix: allow symlink collision error to be downgraded

[mattem]

Introduces an attr allowing the symlink collision errors to be downgraded to a warning or ignored.

Closes #358

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): yes
• Suggested release notes appear below:

Allow the "conflicting symlinks" error when creating a venv to be downgraded to a warning and ignored.

Test plan

• Manual testing; please provide instructions so we can reproduce:

Run //py/tests/virtual/django:manage with the pyqt6 packages installed.