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/psiphonx/psiphonx.go

@@ -0,0 +1,136 @@
+// Package psiphonx is a wrapper around the psiphon-tunnel-core.
+package psiphonx
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"net/url"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+	"github.com/ooni/psiphon/oopsi/github.com/Psiphon-Labs/psiphon-tunnel-core/ClientLibrary/clientlib"
+)
+
+// Session is the way in which this package sees a Session.
+type Session interface {
+	NewOrchestraClient(ctx context.Context) (model.ExperimentOrchestraClient, error)
+	TempDir() string
+}
+
+// Dependencies contains dependencies for Start
+type Dependencies interface {
+	MkdirAll(path string, perm os.FileMode) error
+	RemoveAll(path string) error
+	Start(ctx context.Context, config []byte,
+		workdir string) (*clientlib.PsiphonTunnel, error)
+}
+
+type defaultDependencies struct{}
+
+func (defaultDependencies) MkdirAll(path string, perm os.FileMode) error {
+	return os.MkdirAll(path, perm)
+}
+
+func (defaultDependencies) RemoveAll(path string) error {
+	return os.RemoveAll(path)
+}
+
+func (defaultDependencies) Start(
+	ctx context.Context, config []byte, workdir string) (*clientlib.PsiphonTunnel, error) {
+	return clientlib.StartTunnel(ctx, config, "", clientlib.Parameters{
+		DataRootDirectory: &workdir}, nil, nil)
+}
+
+// Config contains the settings for Start. The empty config object implies
+// that we will be using default settings for starting the tunnel.
+type Config struct {
+	// Dependencies contains dependencies for Start.
+	Dependencies Dependencies
+
+	// WorkDir is the directory where Psiphon should store
+	// its configuration database.
+	WorkDir string
+}
+
+// Tunnel is a psiphon tunnel
+type Tunnel struct {
+	tunnel   *clientlib.PsiphonTunnel
+	duration time.Duration
+}
+
+func makeworkingdir(config Config) (string, error) {
+	const testdirname = "oonipsiphon"
+	workdir := filepath.Join(config.WorkDir, testdirname)
+	if err := config.Dependencies.RemoveAll(workdir); err != nil {
+		return "", err
+	}
+	if err := config.Dependencies.MkdirAll(workdir, 0700); err != nil {
+		return "", err
+	}
+	return workdir, nil
+}
+
+// Start starts the psiphon tunnel.
+func Start(
+	ctx context.Context, sess Session, config Config) (*Tunnel, error) {
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err() // simplifies unit testing this code
+	default:
+	}
+	if config.Dependencies == nil {
+		config.Dependencies = defaultDependencies{}
+	}
+	if config.WorkDir == "" {
+		config.WorkDir = sess.TempDir()
+	}
+	clnt, err := sess.NewOrchestraClient(ctx)
+	if err != nil {
+		return nil, err
+	}
+	configJSON, err := clnt.FetchPsiphonConfig(ctx)
+	if err != nil {
+		return nil, err
+	}
+	workdir, err := makeworkingdir(config)
+	if err != nil {
+		return nil, err
+	}
+	start := time.Now()
+	tunnel, err := config.Dependencies.Start(ctx, configJSON, workdir)
+	if err != nil {
+		return nil, err
+	}
+	stop := time.Now()
+	return &Tunnel{tunnel: tunnel, duration: stop.Sub(start)}, nil
+}
+
+// Stop is an idempotent method that shuts down the tunnel
+func (t *Tunnel) Stop() {
+	if t != nil {
+		t.tunnel.Stop()
+	}
+}
+
+// SOCKS5ProxyURL returns the SOCKS5 proxy URL.
+func (t *Tunnel) SOCKS5ProxyURL() (proxyURL *url.URL) {
+	if t != nil {
+		proxyURL = &url.URL{
+			Scheme: "socks5",
+			Host: net.JoinHostPort(
+				"127.0.0.1", fmt.Sprintf("%d", t.tunnel.SOCKSProxyPort)),
+		}
+	}
+	return
+}
+
+// BootstrapTime returns the bootstrap time
+func (t *Tunnel) BootstrapTime() (duration time.Duration) {
+	if t != nil {
+		duration = t.duration
+	}
+	return
+}