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'`

Bắt đầu với phần thảo luận của issue được liên kết và cấu hình pip.parse sử dụng experimental_index_url, sau đó so sánh hành vi của cquery và genquery được mô tả cho universal uv lockfile. Điều tra đường dẫn Rlocation được tạo và việc cài đặt wheel dành riêng cho nền tảng, bao gồm cả lỗi của pywinpty. Được xem là hoàn tất khi hành vi và một hướng cấu hình hoặc tài liệu phù hợp với cách dùng thông thường được làm rõ cho cả ba vấn đề.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.