fix: pytest >=8.1.0 displays no diff for AssertionError with --import…

…-mode=importlib(pytest-dev#12659)

src/_pytest/pathlib.py

@@ -10,6 +10,7 @@
 import fnmatch
 from functools import partial
 from importlib.machinery import ModuleSpec
+from importlib.machinery import PathFinder
 import importlib.util
 import itertools
 import os
@@ -817,7 +818,10 @@ def resolve_pkg_root_and_module_name(
     if pkg_root is not None:
         module_name = compute_module_name(pkg_root, path)
         if module_name:
-            return pkg_root, module_name
+            spec = PathFinder.find_spec(module_name, [str(pkg_root)])
+            if spec is not None:
+                # Verify that pkk_root and module_name are valid before returning(#12659)
+                return pkg_root, module_name
 
     raise CouldNotResolvePathError(f"Could not resolve for {path}")