Rename migrate_item{,s}_to_target_suite

It accepts multiple items, so using a singular "item" seems like a
misfeature.

Signed-off-by: Niels Thykier <niels@thykier.net>

britney.py

@@ -1475,7 +1475,7 @@ class Britney(object):
                 with mm.start_transaction() as transaction:
                     accepted = False
                     try:
-                        accepted, nuninst_after, failed_arch, new_cruft = mm.migrate_item_to_target_suite(
+                        accepted, nuninst_after, failed_arch, new_cruft = mm.migrate_items_to_target_suite(
                             comp,
                             nuninst_last_accepted
                         )
@@ -1607,7 +1607,7 @@ class Britney(object):
 
             if init:
                 # init => a hint (e.g. "easy") - so do the hint run
-                (_, nuninst_end, _, new_cruft) = mm.migrate_item_to_target_suite(selected,
+                (_, nuninst_end, _, new_cruft) = mm.migrate_items_to_target_suite(selected,
                                                                                   self.nuninst_orig,
                                                                                   stop_on_first_regression=False)
 

britney2/migration.py

@@ -88,7 +88,7 @@ class MigrationManager(object):
         InstallabilityTester.
 
 
-        Unlike migrate_item_to_target_suite, this will not modify
+        Unlike migrate_items_to_target_suite, this will not modify
         any data structure.
         """
         # local copies for better performances
@@ -401,7 +401,7 @@ class MigrationManager(object):
 
         return is_source_migration, affected_architectures, affected_direct, affected_all, smooth_updates
 
-    def migrate_item_to_target_suite(self, items, nuninst_now, stop_on_first_regression=True):
+    def migrate_items_to_target_suite(self, items, nuninst_now, stop_on_first_regression=True):
         is_accepted = True
         target_suite = self.suite_info.target_suite
         packages_t = target_suite.binaries