Issue 4459 - xarfile should not expect Crypto.Signature module to be present

xarfile.py

 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import re
 import struct
 import time
 import zlib
 
 from Crypto.Hash import SHA
 from Crypto.PublicKey import RSA
-from Crypto.Signature import PKCS1_v1_5
 
 from buildtools.packager import getTemplate
 
 XAR_HEADER = struct.Struct('>IHHQQI')
 XAR_HEADER_MAGIC = 0x78617221
 XAR_VERSION = 1
 XAR_CKSUM_SHA1 = 1
 
 
 def read_certificates_and_key(keyfile):
@@ skipping 16 matching lines @@
 
 def get_checksum(data):
     return SHA.new(data).digest()
 
 
 def get_hexchecksum(data):
     return SHA.new(data).hexdigest()
 
 
 def get_signature(key, data):
+    from Crypto.Signature import PKCS1_v1_5
+
     return PKCS1_v1_5.new(key).sign(SHA.new(data))
 
 
 def compress_files(filedata, root, offset):
     compressed_data = []
     filedata = sorted(filedata.iteritems())
     directory_stack = [('', root)]
     file_id = 1
     for path, data in filedata:
         # Remove directories that are done
@@ skipping 72 matching lines @@
                                    len(toc_uncompressed), XAR_CKSUM_SHA1))
 
         # It's followed up with a compressed XML table of contents
         file.write(toc_compressed)
 
         # Now the actual data, all the offsets are in the table of contents
         file.write(get_checksum(toc_compressed))
         file.write(get_signature(key, toc_compressed))
         for blob in compressed_data:
             file.write(blob)