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/dialer/integration_test.go

@@ -9,19 +9,13 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/dialer"
 )
 
-func TestDNSDialerSuccess(t *testing.T) {
+func TestDialerNewSuccess(t *testing.T) {
 	if testing.Short() {
 		t.Skip("skip test in short mode")
 	}
 	log.SetLevel(log.DebugLevel)
-	dialer := dialer.DNSDialer{
-		Dialer: dialer.LoggingDialer{
-			Dialer: new(net.Dialer),
-			Logger: log.Log,
-		},
-		Resolver: new(net.Resolver),
-	}
-	txp := &http.Transport{DialContext: dialer.DialContext}
+	d := dialer.New(&dialer.Config{Logger: log.Log}, &net.Resolver{})
+	txp := &http.Transport{DialContext: d.DialContext}
 	client := &http.Client{Transport: txp}
 	resp, err := client.Get("http://www.google.com")
 	if err != nil {