Custom platform from `pip.default` can't be used by consumers

# 🐞 bug report

## Affected Rule

The issue is caused by the bzlmod module extension pip in the pip.default / pip.parse tag handling in:

- python/private/pypi/extension.bzl (`build_config()`)
- python/private/pypi/parse_requirements.bzl (MacOS, `_package_srcs()`)
- python/private/pypi/hub_builder.bzl (Ubuntu, `_evaluate_markers()`)

### Is this a regression?

No.

### Description

When pip.default() is called from a non-root Bazel module (a dependency module), it is ignored while building the pip extension config. At the same time, pip.parse() in that dependency module is still processed and can create a hub repo that the consumer can use transitively.

This makes it impossible (or very confusing) for a dependency module to ship a reusable pip.parse() hub that relies on custom platforms defined by that same module’s pip.default().

#### Expected behavior

pip.default() platform definitions from a dependency module are applied at least for that module’s own pip.parse() tags.

#### Actual behavior

- pip.default() from dependency modules is ignored.
- pip.parse() from dependency modules is processed.
- If pip.parse() needs a custom platform, evaluation will fail with an error.
- Workaround today: the root module must re-declare the same platforms in its own pip.default().

## 🔬 Minimal Reproduction

Reproduction repository:

## 🔥 Exception or Error

- MacOS: `The target platform 'cp312.12_plat_a' could not be found in ["cp312.12_linux_x86_64", "cp312.12_linux_aarch64", "cp312.12_osx_aarch64", "cp312.12_osx_x86_64", "cp312.12_windows_x86_64", "cp312.12_windows_aarch64"]`
- Linux: `Error: key "cp312.12_plat_a" not found in dictionary`

## 🌍 Your Environment

### Operating System

```text
MacOS 26.1, Ubuntu 24.04
```

### Output of `bazel version`

```text
7.5.0
```

### Rules_python version

```text
1.7.0
```

### Anything else relevant

None.

## References

None.

从 python/private/pypi/extension.bzl 及其对 build_config() 的处理开始，然后跟踪 parse_requirements.bzl（MacOS 的 _package_srcs()）和 hub_builder.bzl（Ubuntu 的 _evaluate_markers()）中的相关逻辑。使用链接的最小复现来验证，依赖模块的 pip.default() 平台会应用于其 pip.parse() hub，而不要求根模块重新声明这些平台。

由索引模型根据 Issue 内容生成。