Close #24748: Restore imp.load_dynamic compatibility

To resolve a compatibility problem found with py2exe and
pywin32, imp.load_dynamic() once again ignores previously loaded modules
to support Python modules replacing themselves with extension modules.

Patch by Petr Viktorin.
diff --git a/Modules/_testmultiphase.c b/Modules/_testmultiphase.c
index 2919687..2005205 100644
--- a/Modules/_testmultiphase.c
+++ b/Modules/_testmultiphase.c
@@ -582,3 +582,13 @@
 {
     return PyModuleDef_Init(&def_exec_unreported_exception);
 }
+
+/*** Helper for imp test ***/
+
+static PyModuleDef imp_dummy_def = TEST_MODULE_DEF("imp_dummy", main_slots, testexport_methods);
+
+PyMODINIT_FUNC
+PyInit_imp_dummy(PyObject *spec)
+{
+    return PyModuleDef_Init(&imp_dummy_def);
+}