BLD: consolidate remaining extensions
moves extensions to ``pandas/libs``, which holds the extension code and also the generated builds (as its importable). ``pandas/src`` is now almost an includes dir, holding low-frequency changing code.

This consolidates the import process making it more uniform and consistent throughout the codebase.

Finally this cleans up the remaining top-level namespace (with some deprecations in place for example ``pandas.lib``, ``pandas.tslib``, ``pandas.json``, ``pandas.parser``. I listed all of the changes in the whatsnew, but I don't think worthwhile deprecating anything else.