Cache _is_setuptools_namespace results to avoid repetitive I/O (#2705)

_is_setuptools_namespace can end up reading the same files over
and over.

For example, when running pylint's import-error checks on yt-dlp,
~23,500 redundant open() calls were performed prior to caching.

Closes pylint-dev/pylint#9603.

Author: correctmost

astroid/interpreter/_import/spec.py

@@ -367,6 +367,7 @@ def contribute_to_path(
 )
 
 
+@lru_cache(maxsize=1024)
 def _is_setuptools_namespace(location: pathlib.Path) -> bool:
     try:
         with open(location / "__init__.py", "rb") as stream:

astroid/manager.py

@@ -439,7 +439,10 @@ def clear_cache(self) -> None:
         # pylint: disable=import-outside-toplevel
         from astroid.brain.helpers import register_all_brains
         from astroid.inference_tip import clear_inference_tip_cache
-        from astroid.interpreter._import.spec import _find_spec
+        from astroid.interpreter._import.spec import (
+            _find_spec,
+            _is_setuptools_namespace,
+        )
         from astroid.interpreter.objectmodel import ObjectModel
         from astroid.nodes._base_nodes import LookupMixIn
         from astroid.nodes.scoped_nodes import ClassDef
@@ -462,6 +465,7 @@ def clear_cache(self) -> None:
             ObjectModel.attributes,
             ClassDef._metaclass_lookup_attribute,
             _find_spec,
+            _is_setuptools_namespace,
         ):
             lru_cache.cache_clear()  # type: ignore[attr-defined]