Remove usages of legacy importlib API.

The Python >= 3.10 compatible usages of importlib added in
277ff54 can be safely applied to any
version of Python >= 3.5 which covers the full range that Hydra
supports.

Getting rid of the old decrecated API, asides from cleaning up, will
keep Hydra compatible with PyInstaller if PyInstaller doesn't get cold
feet and revert its dropping of support for the legacy API
(pyinstaller/pyinstaller#7344).

hydra/core/plugins.py

@@ -182,20 +182,16 @@ def _scan_all_plugins(
                     import_time = timer()
 
                     with warnings.catch_warnings(record=True) as recorded_warnings:
-                        if sys.version_info < (3, 10):
-                            m = importer.find_module(modname)  # type: ignore
-                            assert m is not None
-                            loaded_mod = m.load_module(modname)
+                        spec = importer.find_spec(modname, None)
+                        assert spec is not None
+                        if modname in sys.modules:
+                            loaded_mod = sys.modules[modname]
                         else:
-                            spec = importer.find_spec(modname)
-                            assert spec is not None
-                            if modname in sys.modules:
-                                loaded_mod = sys.modules[modname]
-                            else:
-                                loaded_mod = importlib.util.module_from_spec(spec)
-                            if loaded_mod is not None:
-                                spec.loader.exec_module(loaded_mod)
-                                sys.modules[modname] = loaded_mod
+                            loaded_mod = importlib.util.module_from_spec(spec)
+                        if loaded_mod is not None:
+                            sys.modules[modname] = loaded_mod
+                            assert spec.loader
+                            spec.loader.exec_module(loaded_mod)
 
                     import_time = timer() - import_time
                     if len(recorded_warnings) > 0: