pySim.esim.saip: maintain a parsed fileystem hierarchy

With this change, the ProfileElementSequence object will maintain a
representation of the filesystem hierarchy of the eSIM profile.  Every
file that is added by a ProfileElement will add a FsNode into that tree,
and each FsNode will point to the File object for the respective file.

This allows us to find files by their path, as well as add files by
path.

Change-Id: I2caadc24b1087855f23f3c57cdf8dabbf81757c0

contrib/saip-tool.py

@@ -20,7 +20,7 @@ import sys
 import argparse
 import logging
 import zipfile
-from pathlib import Path
+from pathlib import Path as PlPath
 from typing import List
 
 from pySim.esim.saip import *
@@ -66,7 +66,7 @@ parser_eapp.add_argument('--format', default='cap', choices=['ijc', 'cap'], help
 def do_split(pes: ProfileElementSequence, opts):
     i = 0
     for pe in pes.pe_list:
-        basename = Path(opts.INPUT_UPP).stem
+        basename = PlPath(opts.INPUT_UPP).stem
         if not pe.identification:
             fname = '%s-%02u-%s.der' % (basename, i, pe.type)
         else: