bpo-43445: Add frozen modules to sys.stdlib_module_names (GH-24798)

Add frozen modules to sys.stdlib_module_names. For example, add
"_frozen_importlib" and "_frozen_importlib_external" names.

Add "list_frozen" command to Programs/_testembed.
diff --git a/Programs/_testembed.c b/Programs/_testembed.c
index 52c5674..0901933 100644
--- a/Programs/_testembed.c
+++ b/Programs/_testembed.c
@@ -1721,6 +1721,20 @@ static int test_unicode_id_init(void)
 }
 
 
+// List frozen modules.
+// Command used by Tools/scripts/generate_stdlib_module_names.py script.
+static int list_frozen(void)
+{
+    const struct _frozen *p;
+    for (p = PyImport_FrozenModules; ; p++) {
+        if (p->name == NULL)
+            break;
+        printf("%s\n", p->name);
+    }
+    return 0;
+}
+
+
 
 /* *********************************************************
  * List of test cases and the function that implements it.
@@ -1792,6 +1806,8 @@ static struct TestCase TestCases[] = {
     {"test_audit_run_stdin", test_audit_run_stdin},
 
     {"test_unicode_id_init", test_unicode_id_init},
+
+    {"list_frozen", list_frozen},
     {NULL, NULL}
 };