Strictly define the scope of the public interface of the library

[adubovik]

Currently, pretty much every single module in DIAL SDK is publically available.

E.g. modules utils/*.py may be potentially used by a library user, however, we do not really expect this to happen.

Our expectations should be expressed explicitly in the library code.

The modules/classes/methods which we consider to be private to the library (i.e. pertaining to its implementation details) should be prefixed with underscore, so that they are hidden from the public interface.