chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/internal/tunnel/tunnel.go

@@ -0,0 +1,64 @@
+// 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
+}