Filter to possible package paths before trying to resolve a module.

mypy/modulefinder.py

@@ -416,7 +416,16 @@ def _find_module(self, id: str, use_typeshed: bool) -> ModuleSearchResult:
         found_possible_third_party_missing_type_hints = False
         need_installed_stubs = False
         # Third-party stub/typed packages
-        for pkg_dir in self.search_paths.package_path:
+        candidate_package_dirs = {
+            package_dir[0]
+            for component in (components[0], components[0] + "-stubs")
+            for package_dir in self.find_lib_path_dirs(component, self.search_paths.package_path)
+        }
+        for pkg_dir in [os.path.normpath(p) for p in self.search_paths.package_path]:
+            if pkg_dir not in candidate_package_dirs:
+                if approved_stub_package_exists(id):
+                    need_installed_stubs = True
+                continue
             stub_name = components[0] + "-stubs"
             stub_dir = os_path_join(pkg_dir, stub_name)
             if fscache.isdir(stub_dir):

mypy/test/testmodulefinder.py

@@ -165,7 +165,7 @@ def setUp(self) -> None:
         self.fmc_nons = FindModuleCache(self.search_paths, fscache=None, options=options)
 
     def path(self, *parts: str) -> str:
-        return os.path.join(self.package_dir, *parts)
+        return os.path.normpath(os.path.join(self.package_dir, *parts))
 
     def test__packages_with_ns(self) -> None:
         cases = [