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/system.go

@@ -1,24 +1,12 @@
 package dialer
 
 import (
-	"context"
 	"net"
 	"time"
 )
 
-// underlyingDialer is the underlying net.Dialer.
-var underlyingDialer = &net.Dialer{
+// systemDialer is the underlying net.Dialer.
+var systemDialer = &net.Dialer{
 	Timeout:   15 * time.Second,
 	KeepAlive: 15 * time.Second,
 }
-
-// SystemDialer is the system dialer.
-type SystemDialer struct{}
-
-// DialContext implements Dialer.DialContext
-func (d SystemDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
-	return underlyingDialer.DialContext(ctx, network, address)
-}
-
-// Default is the dialer we use by default.
-var Default = SystemDialer{}