Avoid downloading protobuf dependancy when not needed

Hi we recently migrated a project from workspace to bzlmod and noticed our builds now pull in more things from the internet as a result.
In particular we saw that our builds started to download protobuf (from https://github.com/protocolbuffers/protobuf/releases/download/v29.0-rc2/protobuf-29.0-rc2.zip).
This was despite none of our targets referencing (either directly nor indirectly) `py_proto_library` (or any other protobuf target from `rules_python` or any other module).

It seems that in a workspace project dependencies are fetched lazily [1] whereas in a bzlmod project, dependencies are fetched eagerly.

Reading the source of this library, I saw a comment in the MODULE.bzl https://github.com/bazelbuild/rules_python/blob/29fdcc2c292d64e7c66746d1c701ab67aa05c79b/MODULE.bazel#L12 that suggests the `protobuf` dep will be loaded only if the `py_proto_library` is used. However, that doesn't match my tests where `protobuf` is download by all projects that depend on `rules_python` even if they do not use _any_ rules from it [2].

I am wondering

1. if there is a way to use `rules_python` without pulling `protobuf` if we do not need protobuf support?
2. If the comment linked above needs updating/removing?

#### Related

I think https://github.com/bazelbuild/rules_python/issues/2498 is similar although that is about rules_kotlin and is more about toolchains fetched as module extensions rather than modules downloaded directly.

#### Notes

1: https://bazel.build/extending/repo#when_is_the_implementation_function_executed
2: You can test this by creating a custom registry (https://bazel.build/external/registry) and modifying the `source.json` file of all the protobuf modules to set the `url` field to something that does not exist. Building a bzlmod project that uses rules_python but does not have any `py_proto_library` targets will then fail showing that bazel does try to fetch protobuf.

Commencez par lire MODULE.bzl autour de la ligne 12 et retracez la manière dont la dépendance protobuf est liée à py_proto_library avec bzlmod. Reproduisez le comportement avec un registre personnalisé dont les URL des sources de protobuf sont invalides, en utilisant un projet qui dépend de rules_python mais ne définit aucune cible protobuf. Le travail est considéré comme terminé lorsque le projet n’essaie plus de récupérer protobuf sauf si la prise en charge correspondante est utilisée, et que le commentaire ou la documentation correspond au comportement.

Rédigé par le modèle d'indexation à partir du texte de l'issue.