fix(bzlmod): silence duplicate version registrations (bazel-contrib#2111)

Before this PR we would warn when non-root modules register interpreter
versions and the owners of root modules would have no way to silence the
warning except for patching rules_python.

This PR reduces the default verbosity of the warning to INFO to avoid
spamming users by default.

Fixes bazel-contrib#1818.

Author: aignas

CHANGELOG.md

@@ -55,6 +55,10 @@ A brief description of the categories of changes:
   [#2091](https://github.com/bazelbuild/rules_python/issues/2090).
 * (gazelle) Make `gazelle_python_manifest.update` manual to avoid unnecessary
   network behavior. 
+* (bzlmod): The conflicting toolchains during `python` extension will no longer
+  cause warnings by default. In order to see the warnings for diagnostic purposes
+  set the env var `RULES_PYTHON_REPO_DEBUG_VERBOSITY` to one of `INFO`, `DEBUG` or `TRACE`.
+  Fixes [#1818](https://github.com/bazelbuild/rules_python/issues/1818).
 
 ### Added
 * (rules) `PYTHONSAFEPATH` is inherited from the calling environment to allow

python/private/BUILD.bazel

@@ -132,6 +132,7 @@ bzl_library(
     srcs = ["python.bzl"],
     deps = [
         ":pythons_hub_bzl",
+        ":repo_utils_bzl",
         ":toolchains_repo_bzl",
         ":util_bzl",
         "//python:repositories_bzl",

python/private/python.bzl

@@ -17,6 +17,7 @@
 load("@bazel_features//:features.bzl", "bazel_features")
 load("//python:repositories.bzl", "python_register_toolchains")
 load("//python:versions.bzl", "TOOL_VERSIONS")
+load("//python/private:repo_utils.bzl", "repo_utils")
 load(":pythons_hub.bzl", "hub_repo")
 load(":text_util.bzl", "render")
 load(":toolchains_repo.bzl", "multi_toolchain_aliases")
@@ -27,12 +28,6 @@ load(":util.bzl", "IS_BAZEL_6_4_OR_HIGHER")
 _MAX_NUM_TOOLCHAINS = 9999
 _TOOLCHAIN_INDEX_PAD_LENGTH = len(str(_MAX_NUM_TOOLCHAINS))
 
-# Printing a warning msg not debugging, so we have to disable
-# the buildifier check.
-# buildifier: disable=print
-def _print_warn(msg):
-    print("WARNING:", msg)
-
 def _python_register_toolchains(name, toolchain_attr, module, ignore_root_user_error):
     """Calls python_register_toolchains and returns a struct used to collect the toolchains.
     """
@@ -71,6 +66,8 @@ def _python_impl(module_ctx):
 
     ignore_root_user_error = None
 
+    logger = repo_utils.logger(module_ctx, "python")
+
     # if the root module does not register any toolchain then the
     # ignore_root_user_error takes its default value: False
     if not module_ctx.modules[0].tags.toolchain:
@@ -131,11 +128,14 @@ def _python_impl(module_ctx):
                 # version that rules_python provides as default.
                 first = global_toolchain_versions[toolchain_version]
                 if mod.name != "rules_python" or not first.module.is_root:
+                    # The warning can be enabled by setting the verbosity:
+                    # env RULES_PYTHON_REPO_DEBUG_VERBOSITY=INFO bazel build //...
                     _warn_duplicate_global_toolchain_version(
                         toolchain_version,
                         first = first,
                         second_toolchain_name = toolchain_name,
                         second_module_name = mod.name,
+                        logger = logger,
                     )
                 toolchain_info = None
             else:
@@ -231,11 +231,11 @@ def _fail_duplicate_module_toolchain_version(version, module):
         module = module,
     ))
 
-def _warn_duplicate_global_toolchain_version(version, first, second_toolchain_name, second_module_name):
-    _print_warn((
+def _warn_duplicate_global_toolchain_version(version, first, second_toolchain_name, second_module_name, logger):
+    logger.info(lambda: (
         "Ignoring toolchain '{second_toolchain}' from module '{second_module}': " +
         "Toolchain '{first_toolchain}' from module '{first_module}' " +
-        "already registered Python version {version} and has precedence"
+        "already registered Python version {version} and has precedence."
     ).format(
         first_toolchain = first.name,
         first_module = first.module.name,