Now that we have properly refactored the caching resolvers we can
move them into netxlite as optional resolvers created using the
proper abstract factories we just added.
This diff reduces the complexity and the code size of netx.
See https://github.com/ooni/probe/issues/2121.
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
