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/engine/legacy/netx/resolver.go

@@ -118,7 +118,8 @@ func newResolver(
 	if network == "udp" {
 		// Same rationale as above: avoid possible endless loop
 		return newResolverWrapper(beginning, handler, newResolverUDP(
-			newDialer(beginning, handler), withPort(address, "53"),
+			netxlite.NewDialerLegacyAdapter(newDialer(beginning, handler)),
+			withPort(address, "53"),
 		)), nil
 	}
 	return nil, errors.New("resolver.New: unsupported network value")