Use path file instead of `sitecustomize.py` (#7161)
## Summary
Use a path file (`.pth`) instead of `sitecustomize.py` for configuring
path in emphemeral virtualenvs, overlaying the ephemeral venv on top of
the base `.venv`.
`sitecustomize.py` is a module in the python installation and as such a
unique resource - homebrew pythons on macos already install such a file
and thus uv's `sitecustomize.py`, placed in the ephemeral env, did not
have any effect.
I don't find any documentation explicitly saying that addsitedir is
valid in `.pth` files but from trial it seems to be - and there is the
precedent of the existing _virtualenv.pth _virtualenv.py pair that do
nontrivial operations.
## Test Plan
- Testing on ephemeral venv, resolving to base venv including editable
install in base: done (py3.7, 3.12)
- Testing on homebrew python/macos: done (py3.11)
- tests: run_editable
Fixes #7152
