Avoid direct imports from C extensions.

A step toward PyPy support.

src/ZODB/fsIndex.py

@@ -40,7 +40,7 @@
 # integers.
 import struct
 
-from BTrees._fsBTree import fsBucket
+from BTrees.fsBTree import fsBucket
 from BTrees.OOBTree import OOBTree
 import six
 

src/ZODB/tests/testCache.py

@@ -18,8 +18,8 @@ purposes. It acts like a memo for unpickling.  It also keeps recent
 objects in memory under the assumption that they may be used again.
 """
 
-from persistent.cPickleCache import PickleCache
 from persistent import Persistent
+from persistent import PickleCache
 from persistent.mapping import PersistentMapping
 from ZODB.tests.MinPO import MinPO
 from ZODB.utils import p64