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)

Beginne mit der modules_mapping-Regel und dem Manifest-Generator in gazelle/manifest/manifest.go und konzentriere dich darauf, wie Wheel-Quellen generierten Labels zugeordnet werden. Verwende das Beispiel mit geschichteten Requirements, um die verlorengegangenen Repository-Informationen nachzuverfolgen; abgeschlossen ist die Aufgabe, wenn Imports aus verschiedenen pip-Repositories Labels wie @pip.test//python_dotenv statt immer @pip// erzeugen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.