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_capi.py b/Lib/test/test_capi.py
index 5f5c0d0..8e92a50 100644
--- a/Lib/test/test_capi.py
+++ b/Lib/test/test_capi.py
@@ -581,7 +581,7 @@ def test_fatal_error(self):
         not_expected = ('_testcapi',)
         code = textwrap.dedent('''
             import _testcapi, sys
-            sys.module_names = frozenset({"_testcapi"})
+            sys.stdlib_module_names = frozenset({"_testcapi"})
             _testcapi.fatal_error(b"MESSAGE")
         ''')
         self.check_fatal_error(code, expected)