bpo-46608: exclude marshalled-frozen data if deep-freezing to save 300 KB space (GH-31074)

This reduces the size of the data segment by **300 KB** of the executable because if the modules are deep-frozen then the marshalled frozen data just wastes space. This was inspired by comment by @gvanrossum in https://github.com/python/cpython/pull/29118#issuecomment-958521863. Note: There is a new option `--deepfreeze-only` in `freeze_modules.py` to change this behavior, it is on be default to save disk space.
```console 
# du -s ./python before
27892   ./python
# du -s ./python after
27524   ./python
```

Automerge-Triggered-By: GH:ericsnowcurrently

Include/cpython/import.h

@@ -32,6 +32,7 @@ struct _frozen {
     const char *name;                 /* ASCII encoded string */
     const unsigned char *code;
     int size;
+    bool is_package;
     PyObject *(*get_code)(void);
 };