Don't use Python 3 stdlib stubs in Python 2 mode (#10422)

Previously we could find stubs both in `stdlib/@python2` and
`stdlib` for the same top-level package, which could result in a
mix of Python 2 and 3 stubs being used.

Now if we find stubs for a package in the `@python2` directory, we
skip the parent directory.

Fixes #10421.

Author: JukkaL

mypy/modulefinder.py

@@ -185,6 +185,10 @@ def get_toplevel_possibilities(self, lib_path: Tuple[str, ...], id: str) -> List
                 name = os.path.splitext(name)[0]
                 components.setdefault(name, []).append(dir)
 
+        if self.python2:
+            components = {id: filter_redundant_py2_dirs(dirs)
+                          for id, dirs in components.items()}
+
         self.initial_components[lib_path] = components
         return components.get(id, [])
 
@@ -791,3 +795,19 @@ def typeshed_py_version(options: Options) -> Tuple[int, int]:
         return max(options.python_version, (3, 6))
     else:
         return options.python_version
+
+
+def filter_redundant_py2_dirs(dirs: List[str]) -> List[str]:
+    """If dirs has <dir>/@python2 followed by <dir>, filter out the latter."""
+    if len(dirs) <= 1 or not any(d.endswith(PYTHON2_STUB_DIR) for d in dirs):
+        # Fast path -- nothing to do
+        return dirs
+    seen = []
+    result = []
+    for d in dirs:
+        if d.endswith(PYTHON2_STUB_DIR):
+            seen.append(os.path.dirname(d))
+            result.append(d)
+        elif d not in seen:
+            result.append(d)
+    return result

test-data/unit/python2eval.test

@@ -428,3 +428,8 @@ def foo() -> None:
     reveal_type(x)
 [out]
 _testDefaultDictInference.py:5: note: Revealed type is "collections.defaultdict[builtins.str, builtins.list[builtins.int]]"
+
+[case testIgnorePython3StdlibStubs_python2]
+from collections import abc
+[out]
+_testIgnorePython3StdlibStubs_python2.py:1: error: Module "collections" has no attribute "abc"