MigrationItem: support "$pkg/$ver/$arch" syntax

This relies on the caller supplying a list of valid architectures, and
assumes that none of the list entries are also valid version strings.

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

migrationitem.py

@@ -13,6 +13,16 @@
 # GNU General Public License for more details.
 
 class MigrationItem:
+    _architectures = []
+
+    @classmethod
+    def set_architectures(cls, architectures = None):
+        cls._architectures = architectures or []
+
+    @classmethod
+    def get_architectures(cls):
+        return cls._architectures
+
     def __init__(self, name = None, versionned = False):
         self._name = None
         self._uvname = None
@@ -71,6 +81,10 @@ class MigrationItem:
             else:
                 self._architecture = 'source'
 
+        if self._version in self.__class__.get_architectures():
+            (self._architecture, self._version) = \
+            (self._version, self._architecture)
+
         if '_' in self._architecture:
             self._architecture, self._suite = \
                self._architecture.split('_', 2)