module-policy: update rust extension import to use the new module policy

Differential Revision: https://phab.mercurial-scm.org/D6677

tests/test-rust-discovery.py

@@ -1,16 +1,9 b''
 from __future__ import absolute_import
 import unittest
 
-try:
-    from mercurial import rustext
-    rustext.__name__  # trigger immediate actual import
-except ImportError:
-    rustext = None
-else:
-    # this would fail already without appropriate ancestor.__package__
-    from mercurial.rustext.discovery import (
-        PartialDiscovery,
-    )
+from mercurial import policy
+
+PartialDiscovery = policy.importrust('discovery', member='PartialDiscovery')
 
 try:
     from mercurial.cext import parsers as cparsers
@@ -39,7 +32,7 b' data_non_inlined = ('
     )
 
 
[e-mail](rustext is None or cparsers is None,
[e-mail](PartialDiscovery is None or cparsers is None,
                  "rustext or the C Extension parsers module "
                  "discovery relies on is not available")
 class rustdiscoverytest(unittest.TestCase):