Some questions around `experimental_index_url` behavior

Opening a ticket to dump in all questions for triage, per suggestion of Slack message.

In this thread https://github.com/bazel-contrib/rules_python/issues/2949#issuecomment-3448011820 it was suggested that `experimental_index_url` would fix any issues I was seeing with `RULES_PYTHON_ENABLE_PIPSTAR=1`. That turned out to be true, but I saw a few things that I wanted clarification on.

### Setup

We use a "universal" lock-file generated via `uv` that looks like this:

```
triton==3.4.0 ; python_full_version < '3.13' and platform_machine == 'x86_64' and sys_platform == 'linux' \
...
```

and then in `pip.parse` we don't specify requirements_by_platform as a result:

```python
pip.parse(
enable_implicit_namespace_pkgs = True,
experimental_requirement_cycles = { ... bunch of cycles ... },
hub_name = "core_py_deps",
python_version = "3.12.3",
requirements_lock = "//:3rdparty_python_requirements.txt",
)
```

### Questions/Concerns from the switch

Switching allowed my `genquery` based tests to work alongside `pipstar`, but I was hoping to get clarification on the following:

* One of my tests had this line `r.Rlocation("rules_python++pip+core_py_deps_312_attrs/attrs-25.4.0-py3-none-any.whl")` which I had to switch to `r.Rlocation("rules_python++pip+core_py_deps_312_attrs_py3_none_any_adcf7e2a/attrs-25.4.0-py3-none-any.whl")`. I found the new path by manually `cquery`ing in CLI and am not sure why this is happening or what the idiomatic path forward is. Interestingly this new path works on both OSX and Linux.
* On the branch with `experimental_index_url`, I noticed it was downloading all the linux-only torch/cuda wheels. I don't *think* this happens normally, but I am not an expert. The build still works on Mac, but this is still a little concerning since those wheels are massive. Am I thinking about this the wrong way? I can confirm that in a different branch output_base, the `external` folder doesn't have non OSX wheels. But in the `experimental_index_url` branch, it does.
* `bazel query deps(...)` now fails because it is trying to install a Windows-only package (pywinpty). That package is in my universal lockfile with `os_name == 'nt'`

リンクされた issue の議論と experimental_index_url を使用する pip.parse の設定から始め、次に universal uv lockfile について説明されている cquery と genquery の挙動を比較します。生成された Rlocation パスとプラットフォーム固有の wheel のインストールを調査し、pywinpty の失敗も含めます。3 つすべての懸念事項について、挙動と慣用的な設定またはドキュメントの方針が明確になれば完了です。

索引モデルが issue の本文から書いたものです。