[CLN] avoid internal usage of private functions

[jbrockmendel]

It would be easier to reason about the code if we did a better job of respecting cross-module private-ness. e.g. com._default_index, com._maybe_box_datetimelike, com._values_from_object get used a lot, should either not be used or (more likely) should be de-privatized.

[bashtage]

Maybe these should be in private modules, e.g., _common rather than private functions.

[jreback]

yes these could be moved underneath

_libs (though which is all cython atm)