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/internal/tunnel/tunnel.go

@@ -1,64 +0,0 @@
-// Package tunnel contains code to create a psiphon or tor tunnel.
-package tunnel
-
-import (
-	"context"
-	"errors"
-	"net/url"
-	"time"
-
-	"github.com/ooni/probe-cli/v3/internal/engine/internal/psiphonx"
-	"github.com/ooni/probe-cli/v3/internal/engine/internal/torx"
-	"github.com/ooni/probe-cli/v3/internal/engine/model"
-)
-
-// Session is the way in which this package sees a Session.
-type Session interface {
-	psiphonx.Session
-	torx.Session
-	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 := psiphonx.Start(ctx, config.Session, psiphonx.Config{
-			WorkDir: config.WorkDir,
-		})
-		return enforceNilContract(tun, err)
-	case "tor":
-		logger.Infof("starting %s tunnel; please be patient...", config.Name)
-		tun, err := torx.Start(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
-}