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/experiment/hhfm/hhfm.go

@@ -18,7 +18,6 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/urlgetter"
 	"github.com/ooni/probe-cli/v3/internal/engine/httpheader"
 	"github.com/ooni/probe-cli/v3/internal/engine/model"
-	"github.com/ooni/probe-cli/v3/internal/engine/netx"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/archival"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/dialer"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/errorx"
@@ -312,7 +311,7 @@ type JSONHeaders struct {
 // guarantee that the connection is used for a single request and that
 // such a request does not contain any body.
 type Dialer struct {
-	Dialer  netx.Dialer // used for testing
+	Dialer  dialer.Dialer // used for testing
 	Headers map[string]string
 }
 
@@ -321,7 +320,9 @@ type Dialer struct {
 func (d Dialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
 	child := d.Dialer
 	if child == nil {
-		child = dialer.Default
+		// TODO(bassosimone): figure out why using dialer.New here
+		// causes the experiment to fail with eof_error
+		child = &net.Dialer{Timeout: 15 * time.Second}
 	}
 	conn, err := child.DialContext(ctx, network, address)
 	if err != nil {

internal/engine/experiment/hhfm/hhfm_test.go

@@ -13,7 +13,6 @@ import (
 
 	"github.com/apex/log"
 	"github.com/google/go-cmp/cmp"
-	engine "github.com/ooni/probe-cli/v3/internal/engine"
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/hhfm"
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/urlgetter"
 	"github.com/ooni/probe-cli/v3/internal/engine/internal/mockable"
@@ -55,7 +54,7 @@ func TestSuccess(t *testing.T) {
 		t.Fatal("invalid Agent")
 	}
 	if tk.Failure != nil {
-		t.Fatal("invalid Failure")
+		t.Fatal("invalid Failure", *tk.Failure)
 	}
 	if len(tk.Requests) != 1 {
 		t.Fatal("invalid Requests")
@@ -557,25 +556,6 @@ func TestTransactCannotReadBody(t *testing.T) {
 	}
 }
 
-func newsession(t *testing.T) model.ExperimentSession {
-	sess, err := engine.NewSession(context.Background(), engine.SessionConfig{
-		AvailableProbeServices: []model.Service{{
-			Address: "https://ams-pg-test.ooni.org",
-			Type:    "https",
-		}},
-		Logger:          log.Log,
-		SoftwareName:    "ooniprobe-engine",
-		SoftwareVersion: "0.0.1",
-	})
-	if err != nil {
-		t.Fatal(err)
-	}
-	if err := sess.MaybeLookupBackends(); err != nil {
-		t.Fatal(err)
-	}
-	return sess
-}
-
 func TestTestKeys_FillTampering(t *testing.T) {
 	type fields struct {
 		Agent      string