Don't ask to install a stub package if stubs are installed

If we encounter an import of a submodule of a package with installed
stubs, and the submodule doesn't exist, don't ask to install stubs
since that's not going to help. Also make it possible to ignore this
error using `--ignore-missing-imports`.

Work on #10645.

Author: JukkaL

mypy/build.py

@@ -2447,13 +2447,14 @@ def find_module_and_diagnose(manager: BuildManager,
         # Don't honor a global (not per-module) ignore_missing_imports
         # setting for modules that used to have bundled stubs, as
         # otherwise updating mypy can silently result in new false
-        # negatives.
+        # negatives. (Unless there are stubs but they are incomplete.)
         global_ignore_missing_imports = manager.options.ignore_missing_imports
         py_ver = options.python_version[0]
         if ((is_legacy_bundled_package(top_level, py_ver)
                 or is_legacy_bundled_package(second_level, py_ver))
                 and global_ignore_missing_imports
-                and not options.ignore_missing_imports_per_module):
+                and not options.ignore_missing_imports_per_module
+                and result is ModuleNotFoundReason.APPROVED_STUBS_NOT_INSTALLED):
             ignore_missing_imports = False
 
         if skip_diagnose:

mypy/modulefinder.py

@@ -232,8 +232,11 @@ def _find_module_non_stub_helper(self, components: List[str],
                 plausible_match = True
         if (is_legacy_bundled_package(components[0], self.python_major_ver)
                 or is_legacy_bundled_package('.'.join(components[:2]), self.python_major_ver)):
-            return ModuleNotFoundReason.APPROVED_STUBS_NOT_INSTALLED
-        elif plausible_match:
+            if (len(components) == 1
+                    or (self.find_module(components[0]) is
+                        ModuleNotFoundReason.APPROVED_STUBS_NOT_INSTALLED)):
+                return ModuleNotFoundReason.APPROVED_STUBS_NOT_INSTALLED
+        if plausible_match:
             return ModuleNotFoundReason.FOUND_WITHOUT_TYPE_HINTS
         else:
             return ModuleNotFoundReason.NOT_FOUND