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/integration_test.go

@@ -0,0 +1,53 @@
+package netx
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/bytecounter"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	"github.com/ooni/probe-cli/v3/internal/tracex"
+)
+
+func TestHTTPTransportWorkingAsIntended(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
+	log.SetLevel(log.DebugLevel)
+	counter := bytecounter.New()
+	config := Config{
+		BogonIsError:        true,
+		ByteCounter:         counter,
+		CacheResolutions:    true,
+		ContextByteCounting: true,
+		Logger:              log.Log,
+		ReadWriteSaver:      &tracex.Saver{},
+		Saver:               &tracex.Saver{},
+	}
+	txp := NewHTTPTransport(config)
+	client := &http.Client{Transport: txp}
+	resp, err := client.Get("https://www.google.com")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err = netxlite.ReadAllContext(context.Background(), resp.Body); err != nil {
+		t.Fatal(err)
+	}
+	if err = resp.Body.Close(); err != nil {
+		t.Fatal(err)
+	}
+	if counter.Sent.Load() <= 0 {
+		t.Fatal("no bytes sent?!")
+	}
+	if counter.Received.Load() <= 0 {
+		t.Fatal("no bytes received?!")
+	}
+	if ev := config.ReadWriteSaver.Read(); len(ev) <= 0 {
+		t.Fatal("no R/W events?!")
+	}
+	if ev := config.Saver.Read(); len(ev) <= 0 {
+		t.Fatal("no non-I/O events?!")
+	}
+}