[REF] implement internals as dir (pandas-dev#21903)

* implement internals as dir

* move internals.py to internals/__init__.py unchanged

Author: jbrockmendel

pandas/core/internals.py renamed to pandas/core/internals/__init__.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 import warnings
 import copy
 from warnings import catch_warnings

pandas/tests/internals/test_internals.py

@@ -13,11 +13,12 @@
 from pandas import (Index, MultiIndex, DataFrame, DatetimeIndex,
                     Series, Categorical, TimedeltaIndex, SparseArray)
 from pandas.compat import OrderedDict, lrange
-from pandas.core.internals import (BlockPlacement, SingleBlockManager,
+from pandas.core.internals import (SingleBlockManager,
                                    make_block, BlockManager)
 import pandas.core.algorithms as algos
 import pandas.util.testing as tm
 import pandas as pd
+from pandas._libs.internals import BlockPlacement
 from pandas.util.testing import (assert_almost_equal, assert_frame_equal,
                                  randn, assert_series_equal)
 from pandas.compat import zip, u