convert: make subversion import transport locally

Introducing "absolute_import" feature in previous patch caused failure
of importing local module "transport".

hgext/convert/subversion.py

@@ -52,7 +52,7 b' try:'
     import svn.core
     import svn.ra
     import svn.delta
-    import transport
+    from . import transport
     import warnings
     warnings.filterwarnings('ignore',
             module='svn.core',