Platform-specific packages missing from the unified `@pypi` hub

Start at the pip.parse entry point and reproduce the issue with MODULE.bazel, BUILD.bazel, requirements_linux.txt, and requirements_windows.txt. Compare the concrete @pypi_b//six alias with @pypi//six, then add regression coverage showing that the unified hub includes platform-specific packages and the alias resolves on Linux.

🐞 bug report

(This was diagnosed with help from Codex.)

Affected Rule

pip.parse and the automatically generated unified @pypi hub.

Is this a regression?

Not sure.

Description

I'm seeing an issue where packages that appear only in some platform-specific requirements files are omitted from the unified @pypi hub.

I have an example where I expect @pypi//six to resolve on Linux when the selected concrete hub provides six for Linux. Instead, the unified hub omits the package entirely, even though the concrete hub provides the alias.

The unified hub collects packages from exposed_packages, which filters out platform-specific packages. I think it should collect packages from each concrete hub's complete wheel map and let that hub's aliases handle platform selection.

🔬 Minimal Reproduction

With rules_python and a Python 3.11 toolchain configured, add:

# MODULE.bazel
pip = use_extension("@rules_python//python/extensions:pip.bzl", "pip")

pip.parse(
    hub_name = "pypi_b",
    python_version = "3.11",
    requirements_by_platform = {
        "//:requirements_linux.txt": "linux_*",
        "//:requirements_windows.txt": "windows_*",
    },
    target_platforms = [
        "linux_x86_64",
        "windows_x86_64",
    ],
)

pip.default(default_hub = "pypi_b")

use_repo(pip, "pypi_b", "pypi")

# BUILD.bazel
exports_files([
    "requirements_linux.txt",
    "requirements_windows.txt",
])

# requirements_linux.txt
colorama==0.4.5
six==1.16.0

# requirements_windows.txt
colorama==0.4.5

On Linux x86_64, compare:

bazel build @pypi_b//six
bazel build @pypi//six

The concrete alias resolves, but the unified alias is absent.

🔥 Exception or Error

Bazel cannot resolve @pypi//six because the unified repository does not contain the six package.

🌍 Your Environment

Operating system: linux_x86_64.

Bazel version used to validate the fix: 9.1.1.

rules_python @ 9cc620ebf92d9afaa1c0535968d82d5a7d7e4522.

Anything else relevant?

Using the concrete hub label avoids the missing unified alias. I'd be happy to contribute the fix and regression coverage.