gazelle_python.yaml can only use one pip_repository

The `modules_mapping` rule accepts a list of wheels, but then the [manifest generator](https://github.com/bazelbuild/rules_python/blob/7a437cc94ff722d8c088ab6140b0cd940d28c4d7/gazelle/manifest/manifest.go#L150) assumes that all those modules come from a single `pip.parse` call.

Since many Python projects separate dependencies, i.e. https://pip-tools.readthedocs.io/en/stable/#workflow-for-layered-requirements
it's often the case that some users have different requirements repositories, even for the same folder.

For example, in https://github.com/alexeagle/bazel_python_repro/blob/18f4504adcdef9187f68397ea20a4f6672badcc2/app/BUILD.bazel gazelle has generated line 28 as `@pip//python_dotenv`. However that wheel came from [test.in](https://github.com/alexeagle/bazel_python_repro/blob/18f4504adcdef9187f68397ea20a4f6672badcc2/requirements/test.in#L4) installed at [pip.test](https://github.com/alexeagle/bazel_python_repro/blob/18f4504adcdef9187f68397ea20a4f6672badcc2/MODULE.bazel#L28) - so it should have generated `@pip.test//python_dotenv`.
Of course since the wheels were combined in the [modules_mapping]( https://github.com/alexeagle/bazel_python_repro/blob/18f4504adcdef9187f68397ea20a4f6672badcc2/BUILD.bazel#L31) this information is lost and the `gazelle_python.yaml` can't indicate which repo should be used for a given import (no matter what folder the gazelle_python.yaml were to appear in)

Start with the modules_mapping rule and the manifest generator at gazelle/manifest/manifest.go, focusing on how wheel sources are associated with generated labels. Use the layered requirements example to trace the lost repository information; done means imports from different pip repositories generate labels such as @pip.test//python_dotenv rather than always using @pip//.

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