perf: avoid O(n) packages_distributions() scan at import time

Replace `importlib.metadata.packages_distributions()` with targeted
O(1) `metadata.distribution()` lookups by trying common distribution
name conventions (dot-to-dash, dot-to-underscore, last component).

`packages_distributions()` scans every installed package in the
environment to build a complete module-to-distribution mapping. In
large venvs (500+ packages, common with many google-cloud-* libs),
this causes multi-second import delays for google.api_core and
every library that depends on it.

Also defer the package label resolution in `check_python_version()`
so it only runs when a warning is actually emitted (not on the
common happy path of a supported Python version).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bonauer-pf

packages/google-api-core/google/api_core/_python_version_support.py

@@ -16,6 +16,7 @@
 
 import datetime
 import enum
+from importlib import metadata
 import logging
 import warnings
 import sys
@@ -139,37 +140,33 @@ def _flatten_message(text: str) -> str:
     return " ".join(textwrap.dedent(text).strip().split())
 
 
-# TODO(https://github.com/googleapis/python-api-core/issues/835):
-# Remove once we no longer support Python 3.9.
-# `importlib.metadata.packages_distributions()` is only supported in Python 3.10 and newer
-# https://docs.python.org/3/library/importlib.metadata.html#importlib.metadata.packages_distributions
-if sys.version_info < (3, 10):
+def _get_pypi_package_name(module_name):
+    """Determine the PyPI package name for a given module name.
 
-    def _get_pypi_package_name(module_name):  # pragma: NO COVER
-        """Determine the PyPI package name for a given module name."""
-        return None
-
-else:
-    from importlib import metadata
-
-    def _get_pypi_package_name(module_name):
-        """Determine the PyPI package name for a given module name."""
+    Uses targeted O(1) lookups via importlib.metadata.distribution()
+    instead of packages_distributions(), which does a full O(n) scan
+    of every installed package and is very slow in large environments.
+    """
+    # Try common distribution name conventions. Each
+    # metadata.distribution() call is an O(1) lookup by name.
+    candidates = [
+        module_name.replace(".", "-"),  # google.api_core -> google-api-core
+        module_name.replace(".", "_"),  # google.api_core -> google_api_core
+        module_name.split(".")[-1],  # google.protobuf -> protobuf
+    ]
+    for candidate in candidates:
         try:
-            # Get the mapping of modules to distributions
-            module_to_distributions = metadata.packages_distributions()
-
-            # Check if the module is found in the mapping
-            if module_name in module_to_distributions:  # pragma: NO COVER
-                # The value is a list of distribution names, take the first one
-                return module_to_distributions[module_name][0]
+            return metadata.distribution(candidate).metadata["Name"]
+        except metadata.PackageNotFoundError:
+            continue
         except Exception as e:  # pragma: NO COVER
             _LOGGER.info(
                 "An error occurred while determining PyPI package name for %s: %s",
                 module_name,
                 e,
             )
-
-        return None
+            return None
+    return None
 
 
 def _get_distribution_and_import_packages(import_package: str) -> Tuple[str, Any]:
@@ -195,7 +192,6 @@ def check_python_version(
         The support status of the current Python version.
     """
     today = today or datetime.date.today()
-    package_label, _ = _get_distribution_and_import_packages(package)
 
     python_version = sys.version_info
     version_tuple = (python_version.major, python_version.minor)
@@ -221,7 +217,14 @@ def min_python(date: datetime.date) -> str:
                 return f"{version[0]}.{version[1]}"
         return "at a currently supported version [https://devguide.python.org/versions]"
 
+    # Resolve the pretty package label lazily so we avoid any work on
+    # the happy path (supported Python version, no warning needed).
+    def get_package_label():
+        label, _ = _get_distribution_and_import_packages(package)
+        return label
+
     if gapic_end < today:
+        package_label = get_package_label()
         message = _flatten_message(
             f"""
             You are using a non-supported Python version ({py_version_str}).
@@ -236,6 +239,7 @@ def min_python(date: datetime.date) -> str:
 
     eol_date = version_info.python_eol + EOL_GRACE_PERIOD
     if eol_date <= today <= gapic_end:
+        package_label = get_package_label()
         message = _flatten_message(
             f"""
             You are using a Python version ({py_version_str})
@@ -250,6 +254,7 @@ def min_python(date: datetime.date) -> str:
         return PythonVersionStatus.PYTHON_VERSION_EOL
 
     if gapic_deprecation <= today <= gapic_end:
+        package_label = get_package_label()
         message = _flatten_message(
             f"""
             You are using a Python version ({py_version_str}) which Google will