refactor(netx): reorganize by topic (#800)

Before finishing the ongoing refactoring and leaving whatever
is left of netx in tree, I would like to restructure it so that
we'll have an easy time next time we need to modify it.

Currently, every functionality lives into the `netx.go` file and
we have a support file called `httptransport.go`.

I would like to reorganize by topic, instead. This would allow
future me to more easily perform topic-specific changes.

While there, improve `netx`'s documentation and duplicate some of
this documentation inside `internal/README.md` to provide pointers
to previous documentation, historical context, and some help to
understand the logic architecture of network extensions (aka `netx`).

Part of https://github.com/ooni/probe-cli/pull/396

internal/engine/netx/http.go

@@ -0,0 +1,78 @@
+package netx
+
+//
+// HTTPTransport from Config.
+//
+
+import (
+	"crypto/tls"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+)
+
+// NewHTTPTransport creates a new HTTPRoundTripper from the given Config.
+func NewHTTPTransport(config Config) model.HTTPTransport {
+	if config.Dialer == nil {
+		config.Dialer = NewDialer(config)
+	}
+	if config.TLSDialer == nil {
+		config.TLSDialer = NewTLSDialer(config)
+	}
+	if config.QUICDialer == nil {
+		config.QUICDialer = NewQUICDialer(config)
+	}
+	tInfo := allTransportsInfo[config.HTTP3Enabled]
+	txp := tInfo.Factory(httpTransportConfig{
+		Dialer:     config.Dialer,
+		Logger:     model.ValidLoggerOrDefault(config.Logger),
+		QUICDialer: config.QUICDialer,
+		TLSDialer:  config.TLSDialer,
+		TLSConfig:  config.TLSConfig,
+	})
+	// TODO(bassosimone): I am not super convinced by this code because it
+	// seems we're currently counting bytes twice in some cases. I think we
+	// should review how we're counting bytes and using netx currently.
+	txp = config.ByteCounter.MaybeWrapHTTPTransport(txp)                 // WAI with ByteCounter == nil
+	const defaultSnapshotSize = 0                                        // means: use the default snapsize
+	return config.Saver.MaybeWrapHTTPTransport(txp, defaultSnapshotSize) // WAI with Saver == nil
+}
+
+// httpTransportInfo contains the constructing function as well as the transport name
+type httpTransportInfo struct {
+	Factory       func(httpTransportConfig) model.HTTPTransport
+	TransportName string
+}
+
+var allTransportsInfo = map[bool]httpTransportInfo{
+	false: {
+		Factory:       newSystemTransport,
+		TransportName: "tcp",
+	},
+	true: {
+		Factory:       newHTTP3Transport,
+		TransportName: "quic",
+	},
+}
+
+// httpTransportConfig contains configuration for constructing an HTTPTransport.
+type httpTransportConfig struct {
+	Dialer     model.Dialer
+	Logger     model.Logger
+	QUICDialer model.QUICDialer
+	TLSDialer  model.TLSDialer
+	TLSConfig  *tls.Config
+}
+
+// newHTTP3Transport creates a new HTTP3Transport instance.
+func newHTTP3Transport(config httpTransportConfig) model.HTTPTransport {
+	// Rationale for using NoLogger here: previously this code did
+	// not use a logger as well, so it's fine to keep it as is.
+	return netxlite.NewHTTP3Transport(config.Logger, config.QUICDialer, config.TLSConfig)
+}
+
+// newSystemTransport creates a new "system" HTTP transport. That is a transport
+// using the Go standard library with custom dialer and TLS dialer.
+func newSystemTransport(config httpTransportConfig) model.HTTPTransport {
+	return netxlite.NewHTTPTransport(config.Logger, config.Dialer, config.TLSDialer)
+}