Warn when two packages write to the same module (#13437)
We regularly get confusing bug reports where a package sometimes works
and sometimes doesn't and it's not clear to the user why. Ultimately, it
turns out that two packages contain the same module and there is a race
condition when installing the two packages. Usually, it's one of the
opencv-python distributions, but recently it's been z3, too. These error
are completely inscrutable to users.
* https://github.com/astral-sh/uv/issues/10708
* https://github.com/astral-sh/uv/issues/11806
* https://github.com/astral-sh/uv/issues/11659
* https://github.com/astral-sh/uv/issues/13435
* https://github.com/astral-sh/uv/issues/13550
* https://github.com/astral-sh/uv/issues/14030
We now warn for top-level modules (pattern: `<identifier>/__init__.py`)
that collide in a single installation, naming the offending wheels.
Checking for `__init__.py` excludes namespace packages.
Test script:
```
uv venv -q && cargo run -q --profile fast-build pip install --no-progress --link-mode clone opencv-python opencv-contrib-python --no-build --no-deps
uv venv -q && cargo run -q --profile fast-build pip install --no-progress --link-mode copy opencv-python opencv-contrib-python --no-build --no-deps
uv venv -q && cargo run -q --profile fast-build pip install --no-progress --link-mode hardlink opencv-python opencv-contrib-python --no-build --no-deps
uv venv -q && cargo run -q --profile fast-build pip install --no-progress --link-mode symlink opencv-python opencv-contrib-python --no-build --no-deps
```
We currently only catch conflicts in a single installation. Should we
prime the lock database with the site-packages contents, and would that
carry overhead?
