py/{builtinimport,frozenmod}: Rework frozen modules support to support packages.

Now frozen modules is treated just as a kind of VFS, and all operations
performed on it correspond to operations on normal filesystem. This allows
to support packages properly, and potentially also data files.

This change also have changes to rework frozen bytecode modules support to
use the same framework, but it's not finished (and actually may not work,
as older adhox handling of any type of frozen modules is removed).

py/frozenmod.h

@@ -31,3 +31,4 @@ enum {
 };
 
 int mp_find_frozen_module(const char *str, size_t len, void **data);
+mp_import_stat_t mp_frozen_stat(const char *str);