Prefer stdlib modules over same-named modules on sys.path

For example:
`import copy` now finds `copy` instead of `copy.py`.

This worked correctly before in at least some cases if there
was a (more?) complete chain of __init__.py files from cwd
all the way to the location of the `copy.py` module.

Closes pylint-dev/pylint#6535

ChangeLog

@@ -181,6 +181,11 @@ What's New in astroid 2.15.6?
 =============================
 Release date: TBA
 
+* Prefer standard library modules over same-named modules on sys.path. For example
+  ``import copy`` now finds ``copy`` instead of ``copy.py``. Solves ``no-member`` issues.
+
+  Closes pylint-dev/pylint#6535
+
 * Harden ``get_module_part()`` against ``"."``.
 
   Closes pylint-dev/pylint#8749

astroid/interpreter/_import/spec.py

@@ -20,7 +20,7 @@
 from typing import Any, Literal, NamedTuple, Protocol
 
 from astroid.const import PY310_PLUS
-from astroid.modutils import EXT_LIB_DIRS
+from astroid.modutils import STD_LIB_DIRS, EXT_LIB_DIRS
 
 from . import util
 
@@ -157,6 +157,19 @@ def find_module(
                         location=getattr(spec.loader_state, "filename", None),
                         type=ModuleType.PY_FROZEN,
                     )
+                if (
+                    spec
+                    and isinstance(spec.loader, importlib.machinery.SourceFileLoader)
+                    and any(spec.origin.startswith(std_lib) for std_lib in STD_LIB_DIRS)
+                    and not spec.origin.endswith("__init__.py")
+                ):
+                    # Return standard library modules before local modules
+                    # https://github.com/pylint-dev/pylint/issues/6535
+                    return ModuleSpec(
+                        name=modname,
+                        location=spec.origin,
+                        type=ModuleType.PY_SOURCE,
+                    )
             except ValueError:
                 pass
             submodule_path = sys.path

tests/test_modutils.py

@@ -268,6 +268,15 @@ def test_std_lib(self) -> None:
             os.path.realpath(os.path.__file__.replace(".pyc", ".py")),
         )
 
+    def test_std_lib_found_before_same_named_package_on_path(self) -> None:
+        sys.path.insert(0, resources.find("data"))
+        self.addCleanup(sys.path.pop, 0)
+
+        file = modutils.file_from_modpath(["copy"])
+
+        self.assertNotIn("test", file)  # tests/testdata/python3/data/copy.py
+        self.assertTrue(any(stdlib in file for stdlib in modutils.STD_LIB_DIRS))
+
     def test_builtin(self) -> None:
         self.assertIsNone(modutils.file_from_modpath(["sys"]))
 

tests/testdata/python3/data/copy.py

@@ -0,0 +1 @@
+"""fake copy module (unlike email, we need one without __init__.py)"""