Canonicalize target link in venv

[tgeng]

This way the created venv can be stable across sandboxes.

Internally we implement a test daemon that reuses the same venv (and python process) across tests whenever possible. Hence, if the target link is canonicalized, the same venv can be used across different targets.

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Changes are visible to end-users: no

Test plan

• Covered by existing test cases

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