For testability, replace most if-based construction logic with
calls to well-tested factories living in other packages.
While there, acknowledge that a bunch of types could now be private
and make them private, modifying the code to call the public
factories allowing to construct said types instead.
Part of https://github.com/ooni/probe/issues/2121
This diff modifies netx to stop using most netxlite resolver internals
but the internal function that creates a new, unwrapped system resolver,
which will be dealt with in a subsequent pull request.
See https://github.com/ooni/probe/issues/2121
