[package.proto] add canonical_base_url field.

unittests/package_test.py

 #!/usr/bin/env python
 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import copy
 import doctest
 import os
 import subprocess
 import unittest
@@ skipping 366 matching lines @@
 
 
 class TestPackageSpec(MockIOThings, unittest.TestCase):
   def setUp(self):
     super(TestPackageSpec, self).setUp()
 
     self.proto_text = """
 api_version: 1
 project_id: "super_main_package"
 recipes_path: "path/to/recipes"
+canonical_base_url: "https://bobbie.example.com"
 deps {
   project_id: "bar"
   url: "https://repo.com/bar.git"
   branch: "superbar"
   revision: "deadd00d"
 }
 deps {
   project_id: "foo"
   url: "https://repo.com/foo.git"
   branch: "master"
@@ skipping 73 matching lines @@
       self.assertFalse(checkout.called)
 
 
 def load_tests(_loader, tests, _ignore):
   tests.addTests(doctest.DocTestSuite(package))
   return tests
 
 
 if __name__ == '__main__':
   result = unittest.main()