cleanup: remove unnecessary legacy interfaces (#656)

This diff addresses another point of https://github.com/ooni/probe/issues/1956:

> - [ ] observe that we're still using a bunch of private interfaces for common interfaces such as the `Dialer`, so we can get rid of these private interfaces and always use the ones in `model`, which allows us to remove a bunch of legacy wrappers

Additional cleanups may still be possible. The more I cleanup, the more I see
there's extra legacy code we can dispose of (which seems good?).

internal/engine/netx/tlsdialer/saver_test.go

@@ -3,7 +3,6 @@ package tlsdialer_test
 import (
 	"context"
 	"crypto/tls"
-	"net"
 	"reflect"
 	"testing"
 	"time"
@@ -23,9 +22,7 @@ func TestSaverTLSHandshakerSuccessWithReadWrite(t *testing.T) {
 	saver := &trace.Saver{}
 	tlsdlr := &netxlite.TLSDialerLegacy{
 		Config: &tls.Config{NextProtos: nextprotos},
-		Dialer: netxlite.NewDialerLegacyAdapter(
-			dialer.New(&dialer.Config{ReadWriteSaver: saver}, &net.Resolver{}),
-		),
+		Dialer: dialer.New(&dialer.Config{ReadWriteSaver: saver}, netxlite.DefaultResolver),
 		TLSHandshaker: tlsdialer.SaverTLSHandshaker{
 			TLSHandshaker: &netxlite.TLSHandshakerConfigurable{},
 			Saver:         saver,