refactor: merge dnsx and errorsx into netxlite (#517)

When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/dnsx/model/model.go

@@ -1,14 +0,0 @@
-// Package model contains the dnsx model.
-package model
-
-// HTTPSSvc is an HTTPSSvc reply.
-type HTTPSSvc struct {
-	// ALPN contains the ALPNs inside the HTTPS reply
-	ALPN []string
-
-	// IPv4 contains the IPv4 hints.
-	IPv4 []string
-
-	// IPv6 contains the IPv6 hints.
-	IPv6 []string
-}