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/example_test.go

@@ -0,0 +1,30 @@
+package dialer_test
+
+import (
+	"context"
+	"net"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/dialer"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/trace"
+)
+
+func Example() {
+	saver := &trace.Saver{}
+
+	dlr := dialer.New(&dialer.Config{
+		DialSaver:      saver,
+		Logger:         log.Log,
+		ReadWriteSaver: saver,
+	}, &net.Resolver{})
+
+	ctx := context.Background()
+	conn, err := dlr.DialContext(ctx, "tcp", "8.8.8.8:53")
+	if err != nil {
+		log.WithError(err).Fatal("DialContext failed")
+	}
+
+	// ... use the connection ...
+
+	conn.Close()
+}