Merge overlapping --target install directories

[sronix]

What does this PR do?

Fixes #8799.

pip install --target installs wheel contents into separate scheme
directories before moving them into the target. When package and .data
directories overlapped, target handling skipped or replaced one tree depending
on --upgrade.

Merge the scheme directories in a staging directory before applying existing
target replacement behavior. Duplicate files emit a warning, and the later
scheme entry takes precedence. File-directory conflicts fail before target
contents move.

Tests cover nested merges with and without --upgrade, duplicate files, both
file-directory conflict directions, and existing targets.

Tested with:

• .venv/bin/pytest -q tests/functional/test_install.py -k target (15 passed)
• .venv/bin/pre-commit run --all-files

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.

Assisted-by: Claude and Codex