Issue #16421: allow to load multiple modules from the same shared object.

Patch by Václav Šmilauer.

Modules/_testimportmultiple.c

@@ -0,0 +1,57 @@
+/*
+ * C extensions module to test importing multiple modules from one compiled
+ * file (issue16421). This file defines 3 modules (_testimportmodule,
+ * foo, bar), only the first one is called the same as the compiled file.
+ */
+#include<Python.h>
+
+static struct PyModuleDef _testimportmultiple = {
+    PyModuleDef_HEAD_INIT,
+    "_testimportmultiple",
+    "_testimportmultiple doc",
+    -1,
+    NULL,
+    NULL,
+    NULL,
+    NULL,
+    NULL
+};
+
+PyMODINIT_FUNC PyInit__testimportmultiple()
+{
+    return PyModule_Create(&_testimportmultiple);
+}
+
+static struct PyModuleDef _foomodule = {
+    PyModuleDef_HEAD_INIT,
+    "foo",
+    "foo doc",
+    -1,
+    NULL,
+    NULL,
+    NULL,
+    NULL,
+    NULL
+};
+
+PyMODINIT_FUNC PyInit_foo()
+{
+    return PyModule_Create(&_foomodule);
+}
+
+static struct PyModuleDef _barmodule = {
+    PyModuleDef_HEAD_INIT,
+    "bar",
+    "bar doc",
+    -1,
+    NULL,
+    NULL,
+    NULL,
+    NULL,
+    NULL
+};
+
+PyMODINIT_FUNC PyInit_bar(){
+    return PyModule_Create(&_barmodule);
+}
+