bpo-42955: Rename module_names to sys.stdlib_module_names (GH-24332)

* Rename _Py_module_names to _Py_stdlib_module_names.
* Rename Python/module_names.h to Python/stdlib_module_names.h.
diff --git a/Lib/test/test_faulthandler.py b/Lib/test/test_faulthandler.py
index 02077a6..6486244 100644
--- a/Lib/test/test_faulthandler.py
+++ b/Lib/test/test_faulthandler.py
@@ -336,7 +336,7 @@ def test_dump_ext_modules(self):
             import faulthandler
             import sys
             # Don't filter stdlib module names
-            sys.module_names = frozenset()
+            sys.stdlib_module_names = frozenset()
             faulthandler.enable()
             faulthandler._sigsegv()
             """