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/dialer_test.go

@@ -0,0 +1,57 @@
+package dialer
+
+import (
+	"net"
+	"net/url"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
+)
+
+func TestNewCreatesTheExpectedChain(t *testing.T) {
+	saver := &trace.Saver{}
+	dlr := New(&Config{
+		ContextByteCounting: true,
+		DialSaver:           saver,
+		Logger:              log.Log,
+		ProxyURL:            &url.URL{},
+		ReadWriteSaver:      saver,
+	}, &net.Resolver{})
+	shd, ok := dlr.(*shapingDialer)
+	if !ok {
+		t.Fatal("not a shapingDialer")
+	}
+	bcd, ok := shd.Dialer.(*byteCounterDialer)
+	if !ok {
+		t.Fatal("not a byteCounterDialer")
+	}
+	pd, ok := bcd.Dialer.(*proxyDialer)
+	if !ok {
+		t.Fatal("not a proxyDialer")
+	}
+	dnsd, ok := pd.Dialer.(*dnsDialer)
+	if !ok {
+		t.Fatal("not a dnsDialer")
+	}
+	scd, ok := dnsd.Dialer.(*saverConnDialer)
+	if !ok {
+		t.Fatal("not a saverConnDialer")
+	}
+	sd, ok := scd.Dialer.(*saverDialer)
+	if !ok {
+		t.Fatal("not a saverDialer")
+	}
+	ld, ok := sd.Dialer.(*loggingDialer)
+	if !ok {
+		t.Fatal("not a loggingDialer")
+	}
+	ewd, ok := ld.Dialer.(*errorWrapperDialer)
+	if !ok {
+		t.Fatal("not an errorWrappingDialer")
+	}
+	_, ok = ewd.Dialer.(*net.Dialer)
+	if !ok {
+		t.Fatal("not a net.Dialer")
+	}
+}