fix: split on site-packages path segment when reporting conflicts

[mattem]

When expanding wheels via rules_pycross' pycross_wheel_library, the wheels end up in bazel-out, rather than within the runfiles tree like when consuming from rules_python. To account for this, change the delimiter to site-packages/ given that is the actual common ancestor in the path.

This also changes to using the full paths for reporting the conflict to make it easier to see where the incoming conflict is from.

Closes #359

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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): no
• Suggested release notes appear below: yes

Use site-packages/ as path delimiter when reporting venv package conflicts.

Test plan

• Covered by existing test cases
• Manual testing; please provide instructions so we can reproduce:

Run against https://github.com/tgeng/pycross_venv_bug_repro