[python][typeregistry] _typeName/_includes -> cppTypeName/cppIncludes

Underscore prefixed variables in Python are often by convention internal variables
that should not be accessed from outside the class. These attributes are frequently
used in Python-side C++ code generation code and have to be accessed to write code generators.
Linters like pylint complain about the access of these protected attributes.
This changes them to be part of the public interface (by naming convention).