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/geolocate/stun.go

@@ -0,0 +1,92 @@
+package geolocate
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/pion/stun"
+)
+
+type stunClient interface {
+	Close() error
+	Start(m *stun.Message, h stun.Handler) error
+}
+
+type stunConfig struct {
+	Dial     func(network string, address string) (stunClient, error)
+	Endpoint string
+	Logger   Logger
+}
+
+func stunDialer(network string, address string) (stunClient, error) {
+	return stun.Dial(network, address)
+}
+
+func stunIPLookup(ctx context.Context, config stunConfig) (string, error) {
+	config.Logger.Debugf("STUNIPLookup: start using %s", config.Endpoint)
+	ip, err := func() (string, error) {
+		dial := config.Dial
+		if dial == nil {
+			dial = stunDialer
+		}
+		clnt, err := dial("udp", config.Endpoint)
+		if err != nil {
+			return DefaultProbeIP, err
+		}
+		defer clnt.Close()
+		message := stun.MustBuild(stun.TransactionID, stun.BindingRequest)
+		errch, ipch := make(chan error, 1), make(chan string, 1)
+		err = clnt.Start(message, func(ev stun.Event) {
+			if ev.Error != nil {
+				errch <- ev.Error
+				return
+			}
+			var xorAddr stun.XORMappedAddress
+			if err := xorAddr.GetFrom(ev.Message); err != nil {
+				errch <- err
+				return
+			}
+			ipch <- xorAddr.IP.String()
+		})
+		if err != nil {
+			return DefaultProbeIP, err
+		}
+		select {
+		case err := <-errch:
+			return DefaultProbeIP, err
+		case ip := <-ipch:
+			return ip, nil
+		case <-ctx.Done():
+			return DefaultProbeIP, ctx.Err()
+		}
+	}()
+	if err != nil {
+		config.Logger.Debugf("STUNIPLookup: failure using %s: %+v", config.Endpoint, err)
+		return DefaultProbeIP, err
+	}
+	return ip, nil
+}
+
+func stunEkigaIPLookup(
+	ctx context.Context,
+	httpClient *http.Client,
+	logger Logger,
+	userAgent string,
+) (string, error) {
+	return stunIPLookup(ctx, stunConfig{
+		Endpoint: "stun.ekiga.net:3478",
+		Logger:   logger,
+	})
+}
+
+func stunGoogleIPLookup(
+	ctx context.Context,
+	httpClient *http.Client,
+	logger Logger,
+	userAgent string,
+) (string, error) {
+	return stunIPLookup(ctx, stunConfig{
+		Endpoint: "stun.l.google.com:19302",
+		Logger:   logger,
+	})
+}