refactor(netxlite): introduce the getaddrinfo transport (#775)

This diff modifies the system resolver to use a getaddrinf transport.

Obviously the transport is a fake, but its existence will allow us
to observe DNS events more naturally.

A lookup using the system resolver would be a ANY lookup that will
contain all the resolved IP addresses into the same response.

This change was also part of websteps-illustrated, albeit the way in
which I did it there was less clean than what we have here.

Ref issue: https://github.com/ooni/probe/issues/2096

internal/netxlite/dialer_test.go

@@ -123,7 +123,7 @@ func TestDialerResolver(t *testing.T) {
 		t.Run("fails without a port", func(t *testing.T) {
 			d := &dialerResolver{
 				Dialer:   &DialerSystem{},
-				Resolver: &resolverSystem{},
+				Resolver: newResolverSystem(),
 			}
 			const missingPort = "ooni.nu"
 			conn, err := d.DialContext(context.Background(), "tcp", missingPort)