gh-142315: Don't pass the "real path" of Pdb script target to system functions (#142371)

* Pick target depending on preconditions

* Clarify the news fragment

* Add test capturing missed expectation.

* Add more idiomatic safe realpath helper

* Restore logic where existance and directoriness are checked on realpath.

* Link GH issue to test.

* Extract a function to check the target. Remove the _safe_realpath, now no longer needed.

* Extract method for replacing sys_path, and isolate realpath usage there.

* Revert "Extract method for replacing sys_path, and isolate realpath usage there."

This reverts commit 855aac3d289dd096142ed9bd23d00c22ce6e1859.

* Restore _safe_realpath.

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Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>