refactor(netx/dialer): hide implementation complexity (#372)

* refactor(netx/dialer): hide implementation complexity

This follows the blueprint of `module.Config` and `nodule.New`
described at https://github.com/ooni/probe/issues/1591.

* fix: ndt7 bug where we were not using the right resolver

* fix(legacy/netx): clarify irrelevant implementation change

* fix: improve comments

* fix(hhfm): do not use dialer.New b/c it breaks it

Unclear to me why this is happening. Still, improve upon the
previous situation by adding a timeout.

It does not seem a priority to look into this issue now.

internal/engine/netx/tlsdialer/saver_test.go

@@ -23,10 +23,7 @@ func TestSaverTLSHandshakerSuccessWithReadWrite(t *testing.T) {
 	saver := &trace.Saver{}
 	tlsdlr := tlsdialer.TLSDialer{
 		Config: &tls.Config{NextProtos: nextprotos},
-		Dialer: dialer.SaverConnDialer{
-			Dialer: new(net.Dialer),
-			Saver:  saver,
-		},
+		Dialer: dialer.New(&dialer.Config{ReadWriteSaver: saver}, &net.Resolver{}),
 		TLSHandshaker: tlsdialer.SaverTLSHandshaker{
 			TLSHandshaker: tlsdialer.SystemTLSHandshaker{},
 			Saver:         saver,