Refuse wheel installs through directory symlinks and replace file symlinks

[notatallshaw]

What does this PR do?

When pip writes a file, it follows any symlink already at that path, and a dangling symlink at a path pip is about to create is followed too. So a written file can land wherever the link points: wheel members, console scripts, REQUESTED, pip download and pip wheel output, and the --target copy across filesystems.

Fixes:

• Wheel installs now resolve each file's parent directory and refuse when it falls outside its scheme directory.
• A symlinked __pycache__ is refused the same way.
• A symlink at a path pip is about to write is unlinked first, so a regular file replaces it and the link target is untouched.
• A symlink that resolves inside the scheme still works

PR Checklist:

• I agree to follow the PSF Code of Conduct.
• I have read and have followed the CONTRIBUTING.md file.
• I have added a news file fragment (or this PR does not need one).
• I have read and followed the AI_POLICY.md file, and if any AI tools were used, I have disclosed it below.

Used Claude and Codex to do multiple rounds of checking for completeness and correctness.