cleanup(netx): remove subpackages and unnecessary code (#788)

This pull request consists of several small and obvious cleanups in the netx directory.

See https://github.com/ooni/probe/issues/2121

internal/engine/netx/dialer.go

@@ -0,0 +1,63 @@
+package netx
+
+import (
+	"net/url"
+
+	"github.com/ooni/probe-cli/v3/internal/bytecounter"
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	"github.com/ooni/probe-cli/v3/internal/tracex"
+)
+
+// dialerConfig contains the settings for New.
+type dialerConfig struct {
+	// ContextByteCounting optionally configures context-based
+	// byte counting. By default we don't do that.
+	//
+	// Use WithExperimentByteCounter and WithSessionByteCounter
+	// to assign byte counters to a context. The code will use
+	// corresponding, private functions to access the configured
+	// byte counters and will notify them about I/O events.
+	//
+	// Bug
+	//
+	// This implementation cannot properly account for the bytes that are sent by
+	// persistent connections, because they stick to the counters set when the
+	// connection was established. This typically means we miss the bytes sent and
+	// received when submitting a measurement. Such bytes are specifically not
+	// seen by the experiment specific byte counter.
+	//
+	// For this reason, this implementation may be heavily changed/removed.
+	ContextByteCounting bool
+
+	// DialSaver is the optional saver for dialing events. If not
+	// set, we will not save any dialing event.
+	DialSaver *tracex.Saver
+
+	// Logger is the optional logger. If not set, there
+	// will be no logging from the new dialer.
+	Logger model.DebugLogger
+
+	// ProxyURL is the optional proxy URL.
+	ProxyURL *url.URL
+
+	// ReadWriteSaver is like DialSaver but for I/O events.
+	ReadWriteSaver *tracex.Saver
+}
+
+// newDialer creates a new Dialer from the specified config and resolver.
+func newDialer(config *dialerConfig, resolver model.Resolver) model.Dialer {
+	var logger model.DebugLogger = model.DiscardLogger
+	if config.Logger != nil {
+		logger = config.Logger
+	}
+	d := netxlite.NewDialerWithResolver(
+		logger, resolver, config.DialSaver.NewConnectObserver(),
+		config.ReadWriteSaver.NewReadWriteObserver(),
+	)
+	d = &netxlite.MaybeProxyDialer{ProxyURL: config.ProxyURL, Dialer: d}
+	if config.ContextByteCounting {
+		d = &bytecounter.ContextAwareDialer{Dialer: d}
+	}
+	return d
+}