unable to override https://pypi.org/simple starting in rules_python 2.0.0

# 🐞 bug report

### Affected Rule

The issue is caused by the rule:

pip.parse.experimental_index_url
pip.parse.experimental_extra_index_urls
pip.default.index_url

### Is this a regression?

Yes, the previous version in which this bug was not present was: 1.9.1

### Description

In rules_python 1.9.1 and earlier, I was able to override the use of https://pypi.org/simple as follows:
```
pip.parse(
envsubst = ["EXAMPLE_PIP_INDEX_URL"],
experimental_extra_index_urls = [
"${EXAMPLE_PIP_INDEX_URL}",
],
experimental_index_url = "${EXAMPLE_PIP_INDEX_URL}",
...
)
```
where `EXAMPLE_PIP_INDEX_URL` is an environment variable that we set to our local pypi index.
We also have a `--downloader_config` set up which blocks access to `pypi.org`.

Starting with rules_python 2.0.0 and continuing through 2.3.2, the `experimental_extra_index_urls` and `experimental_index_url` parameters silently no longer work, and do not generate any diagnostic to say what the new mechanism is.

Per the documentation at https://rules-python.readthedocs.io/en/2.3.2/api/rules_python/python/extensions/pip.html#pip.default.index_url I would expect the following to work starting with rules_python 2.0.0:
```
pip.default(
index_url = "${EXAMPLE_PIP_INDEX_URL}",
)
pip.parse(
envsubst = ["EXAMPLE_PIP_INDEX_URL"],
...
)
```
However, it appears that `index_url` is not used at all, or at least does not override all uses of https://pypi.org/simple. The docs state that

> This value is going to be subject to envsubst substitutions if necessary

I do need it to be subject to envsubst substitutions. It would be clearer if the docs said

> This value is subject to envsubst substitutions

## 🔬 Minimal Reproduction

I can provide a small reproduction if necessary, but the user would need to supply some alternative to pypi.org to configure it with.

## 🔥 Exception or Error

When I run `bazel clean --expunge; bazel mod graph` in a repo configured as I described above, I get a diagnostic like
```
Error in wait: java.io.IOException: Configured URL rewriter blocked all URLs: [https://pypi.org/simple/]
ERROR: error evaluating module extension @@rules_python+//python/extensions:pip.bzl%pip
ERROR: Results may be incomplete as 1 extension failed.
```
starting with rules_python 2.0.0 due to pypi.org/simple not being overridden any more, either by the old mechanism with `experimental_index_url` and `experimental_extra_index_urls`, or by the new mechanism with `index_url`.

## 🌍 Your Environment

**Operating System:**

Ubuntu 24.04

**Output of `bazel version`:**

  

$ bazelisk version

Bazelisk version: v1.19.0

Build label: 9.2.0

Build target: @@//src/main/java/com/google/devtools/build/lib/bazel:BazelServer

Build time: Mon Jul 13 18:15:04 2026 (1783966504)

Build timestamp: 1783966504

Build timestamp as int: 1783966504

  

**Rules_python version:**

  

2.3.2

  

**Anything else relevant?**

I'd be happy to attempt a fix, given some guidance as to what needed to be done where.
I'm also happy to test prospective fixes if my particular computing environment can be of some help.

It would be nice if `experimental_extra_index_urls` and `experimental_index_url` continued to work. But if they are not going to work, their use should at a minimum trigger some sort of clear diagnostic about what to use instead.

Posted on the bazel slack at https://bazelbuild.slack.com/archives/CA306CEV6/p1787963436682509