gh-39423: Use import_module instead of find_spec

    
Otherwise the test would fail with meson editable install. See https://g
ithub.com/sagemath/sage/actions/runs/13003203795/job/36265539648 .

Looks like the function was last changed in #36407. There was no
discussion why the simple implementation is not used.

This is part of the fix for this test. The other part needed is
#39498


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- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.  (can't really test,
but see #39369)
- [ ] I have updated the documentation and checked the documentation
preview.  (no documentation change)

### ⌛ Dependencies

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URL: #39423
Reported by: user202729
Reviewer(s): Tobias Diez

src/sage/misc/dev_tools.py

@@ -171,7 +171,7 @@ def load_submodules(module=None, exclude_pattern=None):
         load sage.geometry.polyhedron.ppl_lattice_polygon... succeeded
     """
     from .package_dir import walk_packages
-    import importlib.util
+    import importlib
 
     if module is None:
         import sage
@@ -195,12 +195,8 @@ def load_submodules(module=None, exclude_pattern=None):
         try:
             sys.stdout.write("load %s..." % module_name)
             sys.stdout.flush()
-            # see
-            # https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly
-            spec = importer.find_spec(module_name)
-            module = importlib.util.module_from_spec(spec)
-            sys.modules[module_name] = module
-            spec.loader.exec_module(module)
+            module = importlib.import_module(module_name)
+            assert sys.modules[module_name] is module
             sys.stdout.write(" succeeded\n")
         except (ValueError, AttributeError, TypeError, ImportError):
             # we might get error because of cython code that has been