Mark all symbols not supposed to be exposed as private

Figure out the correct way for, and mark all symbols not explicitly supposed to be exposed as private to avoid unintended use and unnecessary coupling.

Check Python conventions, and available technical solutions, to mark module-public symbols which are library-private. I.e. symbols which are used by other modules in the library, but are not supposed to be used outside the library.

Start by reviewing Python conventions for public and private module symbols, then inspect the kcidb library modules to identify symbols used internally but not intended for external callers. Define the marking approach and the complete set of symbols that should be private before making changes; the issue does not name specific files or tests.

Written by the indexing model from the issue text.