migrationitem: apply some fixes to the version-derivation code

The major change is reversing the order of three-part item names (i.e.
binNMUs with version information included) to use the more traditional
ordering of <src>/<arch>/<ver>.

Even if an instance is marked as versionned, passing a non-versionned
source package name should not cause a traceback.

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

migrationitem.py

@@ -37,12 +37,13 @@ class MigrationItem:
             self._package, self._suite = package.split('_', 2)
         else:
             self._package, self._suite = (package, 'unstable')
-        if self._versionned:
-            self._version = parts[1]
+        if self._versionned and len(parts) > 1:
             if len(parts) == 3:
-                self._architecture = parts[2]
+                self._architecture = parts[1]
+                self._version = parts[2]
             else:
                 self._architecture = 'source'
+                self._version = parts[1]
         else:
             if len(parts) == 2:
                 self._architecture = parts[1]