Revert "Use importlib instead of pkg_resources for determining namesp…

…ace packages" (#1409)

This reverts commit 8f477fd.

astroid/interpreter/_import/util.py

@@ -3,70 +3,15 @@
 # Copyright (c) 2021 Daniël van Noord <[email protected]>
 # Copyright (c) 2021 Neil Girdhar <[email protected]>
 
+try:
+    import pkg_resources
+except ImportError:
+    pkg_resources = None  # type: ignore[assignment]
 
-from importlib import abc, util
-
-from astroid.const import PY36
-
-
-def _is_old_setuptools_namespace_package(modname: str) -> bool:
-    """Check for old types of setuptools namespace packages.
-
-    See https://setuptools.pypa.io/en/latest/pkg_resources.html and
-    https://packaging.python.org/en/latest/guides/packaging-namespace-packages/
-
-    Because pkg_resources is slow to import we only do so if explicitly necessary.
-    """
-    try:
-        import pkg_resources  # pylint: disable=import-outside-toplevel
-    except ImportError:
-        return False
 
+def is_namespace(modname):
     return (
-        hasattr(pkg_resources, "_namespace_packages")
-        and modname in pkg_resources._namespace_packages  # type: ignore[attr-defined]
-    )
-
-
-def is_namespace(modname: str) -> bool:
-    """Determine whether we encounter a namespace package."""
-    if PY36:
-        # On Python 3.6 an AttributeError is raised when a package
-        # is lacking a __path__ attribute and thus is not a
-        # package.
-        try:
-            spec = util.find_spec(modname)
-        except (AttributeError, ValueError):
-            return _is_old_setuptools_namespace_package(modname)
-    else:
-        try:
-            spec = util.find_spec(modname)
-        except ValueError:
-            return _is_old_setuptools_namespace_package(modname)
-
-    # If there is no spec or origin this is a namespace package
-    # See: https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.origin
-    # We assume builtin packages are never namespace
-    if not spec or not spec.origin or spec.origin == "built-in":
-        return False
-
-    # If there is no loader the package is namespace
-    # See https://docs.python.org/3/library/importlib.html#importlib.abc.PathEntryFinder.find_loader
-    if not spec.loader:
-        return True
-    # This checks for _frozen_importlib.FrozenImporter, which does not inherit from InspectLoader
-    if hasattr(spec.loader, "_ORIGIN") and spec.loader._ORIGIN == "frozen":
-        return False
-    # Other loaders are namespace packages
-    if not isinstance(spec.loader, abc.InspectLoader):
-        return True
-
-    # Lastly we check if the package declares itself a namespace package
-    try:
-        source = spec.loader.get_source(spec.origin)
-    # If the loader can't handle the spec, we're dealing with a namespace package
-    except ImportError:
-        return False
-    return bool(
-        source and "pkg_resources" in source and "declare_namespace(__name__)" in source
+        pkg_resources is not None
+        and hasattr(pkg_resources, "_namespace_packages")
+        and modname in pkg_resources._namespace_packages
     )