tools/make-frozen.py: Quick fix to support package-modules.

It allows to "import foo.bar", but not "from foo import bar".

tools/make-frozen.py

@@ -40,7 +40,7 @@ print("#include <stdint.h>")
 print("const char mp_frozen_str_names[] = {")
 for f, st in modules:
     m = module_name(f)
-    print('"%s\\0"' % m)
+    print('"%s\\0"' % m.replace("/", "."))
 print('"\\0"};')
 
 print("const uint32_t mp_frozen_str_sizes[] = {")