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/dialer/integration_test.go

@@ -1,12 +1,12 @@
 package dialer_test
 
 import (
-	"net"
 	"net/http"
 	"testing"
 
 	"github.com/apex/log"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/dialer"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func TestDialerNewSuccess(t *testing.T) {
@@ -14,7 +14,7 @@ func TestDialerNewSuccess(t *testing.T) {
 		t.Skip("skip test in short mode")
 	}
 	log.SetLevel(log.DebugLevel)
-	d := dialer.New(&dialer.Config{Logger: log.Log}, &net.Resolver{})
+	d := dialer.New(&dialer.Config{Logger: log.Log}, netxlite.DefaultResolver)
 	txp := &http.Transport{DialContext: d.DialContext}
 	client := &http.Client{Transport: txp}
 	resp, err := client.Get("http://www.google.com")