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_test.go

@@ -0,0 +1,50 @@
+package netx
+
+import (
+	"context"
+	"errors"
+	"net"
+	"net/http"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/model/mocks"
+	"github.com/ooni/probe-cli/v3/internal/tracex"
+)
+
+func TestNewHTTPTransportWithDialer(t *testing.T) {
+	expected := errors.New("mocked error")
+	dialer := &mocks.Dialer{
+		MockDialContext: func(ctx context.Context, network, address string) (net.Conn, error) {
+			return nil, expected
+		},
+	}
+	txp := NewHTTPTransport(Config{
+		Dialer: dialer,
+	})
+	client := &http.Client{Transport: txp}
+	resp, err := client.Get("http://www.google.com")
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if resp != nil {
+		t.Fatal("not the response we expected")
+	}
+}
+
+func TestNewHTTPTransportWithSaver(t *testing.T) {
+	saver := new(tracex.Saver)
+	txp := NewHTTPTransport(Config{
+		Saver: saver,
+	})
+	stxptxp, ok := txp.(*tracex.HTTPTransportSaver)
+	if !ok {
+		t.Fatal("not the transport we expected")
+	}
+	if stxptxp.Saver != saver {
+		t.Fatal("not the logger we expected")
+	}
+	if stxptxp.Saver != saver {
+		t.Fatal("not the logger we expected")
+	}
+	// We are going to trust the underlying type returned by netxlite
+}