refactor: merge psiphonx and torx into tunnel (#287)

* refactor: merge psiphonx and torx into tunnel

This is a case where it seems that merging these three packages into
a single package will enable us to better the implementation.

The goal is still https://github.com/ooni/probe/issues/985.

The roadblock I'm trying to overcome is
https://github.com/ooni/probe-cli/pull/286#pullrequestreview-627460104.

* avoid duplicating logger for now

internal/engine/tunnel/tunnel.go

@@ -0,0 +1,65 @@
+// Package tunnel allows to create tunnels to speak
+// with OONI backends and other services.
+package tunnel
+
+import (
+	"context"
+	"errors"
+	"net/url"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+// Session is the way in which this package sees a Session.
+type Session interface {
+	FetchPsiphonConfig(ctx context.Context) ([]byte, error)
+	TempDir() string
+	TorArgs() []string
+	TorBinary() string
+	Logger() model.Logger
+}
+
+// Tunnel is a tunnel used by the session
+type Tunnel interface {
+	BootstrapTime() time.Duration
+	SOCKS5ProxyURL() *url.URL
+	Stop()
+}
+
+// Config contains config for the session tunnel.
+type Config struct {
+	Name    string
+	Session Session
+	WorkDir string
+}
+
+// Start starts a new tunnel by name or returns an error. Note that if you
+// pass to this function the "" tunnel, you get back nil, nil.
+func Start(ctx context.Context, config Config) (Tunnel, error) {
+	logger := config.Session.Logger()
+	switch config.Name {
+	case "":
+		logger.Debugf("no tunnel has been requested")
+		return enforceNilContract(nil, nil)
+	case "psiphon":
+		logger.Infof("starting %s tunnel; please be patient...", config.Name)
+		tun, err := psiphonStart(ctx, config.Session, psiphonConfig{
+			WorkDir: config.WorkDir,
+		})
+		return enforceNilContract(tun, err)
+	case "tor":
+		logger.Infof("starting %s tunnel; please be patient...", config.Name)
+		tun, err := torStart(ctx, config.Session)
+		return enforceNilContract(tun, err)
+	default:
+		return nil, errors.New("unsupported tunnel")
+	}
+}
+
+func enforceNilContract(tun Tunnel, err error) (Tunnel, error) {
+	if err != nil {
+		return nil, err
+	}
+	return tun, nil
+}