gh-106320: Create pycore_modsupport.h header file (#106355)

Remove the following functions from the C API, move them to the internal C
API: add a new pycore_modsupport.h internal header file:

* PyModule_CreateInitialized()
* _PyArg_NoKwnames()
* _Py_VaBuildStack()

No longer export these functions.

Include/internal/pycore_modsupport.h

@@ -0,0 +1,29 @@
+#ifndef Py_INTERNAL_MODSUPPORT_H
+#define Py_INTERNAL_MODSUPPORT_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+
+extern int _PyArg_NoKwnames(const char *funcname, PyObject *kwnames);
+#define _PyArg_NoKwnames(funcname, kwnames) \
+    ((kwnames) == NULL || _PyArg_NoKwnames((funcname), (kwnames)))
+
+extern PyObject ** _Py_VaBuildStack(
+    PyObject **small_stack,
+    Py_ssize_t small_stack_len,
+    const char *format,
+    va_list va,
+    Py_ssize_t *p_nargs);
+
+extern PyObject* _PyModule_CreateInitialized(PyModuleDef*, int apiver);
+
+#ifdef __cplusplus
+}
+#endif
+#endif  // !Py_INTERNAL_MODSUPPORT_H
+