CLN: hide all imported extensions

[jreback]

so most of these are in the top-level namespace, so let's rename to use _ leading if not already like that.

e.g. _lib rather than lib

just requires some small changes in setup.py, but a fair number of imports need to be changed.
This is a slight API change, in that people could be relying upon it. So we could create a deprecation warning if these names are accessed (and then have it work).

~/pandas]$ find pandas -name '*.so'
pandas/_period.so
pandas/_sparse.so
pandas/_testing.so
pandas/algos.so
pandas/hashtable.so
pandas/index.so
pandas/io/sas/saslib.so
pandas/json.so
pandas/lib.so
pandas/msgpack/_packer.so
pandas/msgpack/_unpacker.so
pandas/parser.so
pandas/tslib.so