Refactor insert_missing_modules function (#12210)

Makes the logic more straightforward IMO.

src/_pytest/pathlib.py

@@ -737,34 +737,31 @@ def insert_missing_modules(modules: Dict[str, ModuleType], module_name: str) ->
     otherwise "src.tests.test_foo" is not importable by ``__import__``.
     """
     module_parts = module_name.split(".")
-    child_module: Union[ModuleType, None] = None
-    module: Union[ModuleType, None] = None
-    child_name: str = ""
     while module_name:
-        if module_name not in modules:
-            try:
-                # If sys.meta_path is empty, calling import_module will issue
-                # a warning and raise ModuleNotFoundError. To avoid the
-                # warning, we check sys.meta_path explicitly and raise the error
-                # ourselves to fall back to creating a dummy module.
-                if not sys.meta_path:
-                    raise ModuleNotFoundError
-                module = importlib.import_module(module_name)
-            except ModuleNotFoundError:
-                module = ModuleType(
-                    module_name,
-                    doc="Empty module created by pytest's importmode=importlib.",
-                )
-        else:
-            module = modules[module_name]
-        if child_module:
+        parent_module_name, _, child_name = module_name.rpartition(".")
+        if parent_module_name:
+            parent_module = modules.get(parent_module_name)
+            if parent_module is None:
+                try:
+                    # If sys.meta_path is empty, calling import_module will issue
+                    # a warning and raise ModuleNotFoundError. To avoid the
+                    # warning, we check sys.meta_path explicitly and raise the error
+                    # ourselves to fall back to creating a dummy module.
+                    if not sys.meta_path:
+                        raise ModuleNotFoundError
+                    parent_module = importlib.import_module(parent_module_name)
+                except ModuleNotFoundError:
+                    parent_module = ModuleType(
+                        module_name,
+                        doc="Empty module created by pytest's importmode=importlib.",
+                    )
+                modules[parent_module_name] = parent_module
+
             # Add child attribute to the parent that can reference the child
             # modules.
-            if not hasattr(module, child_name):
-                setattr(module, child_name, child_module)
-                modules[module_name] = module
-        # Keep track of the child module while moving up the tree.
-        child_module, child_name = module, module_name.rpartition(".")[-1]
+            if not hasattr(parent_module, child_name):
+                setattr(parent_module, child_name, modules[module_name])
+
         module_parts.pop(-1)
         module_name = ".".join(module_parts)