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/legacy/netx/handlers/handlers.go

@@ -0,0 +1,52 @@
+// Package handlers contains default modelx.Handler handlers.
+package handlers
+
+import (
+	"encoding/json"
+	"fmt"
+	"sync"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/runtimex"
+	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/modelx"
+)
+
+type stdoutHandler struct{}
+
+func (stdoutHandler) OnMeasurement(m modelx.Measurement) {
+	data, err := json.Marshal(m)
+	runtimex.PanicOnError(err, "unexpected json.Marshal failure")
+	fmt.Printf("%s\n", string(data))
+}
+
+// StdoutHandler is a Handler that logs on stdout.
+var StdoutHandler stdoutHandler
+
+type noHandler struct{}
+
+func (noHandler) OnMeasurement(m modelx.Measurement) {
+}
+
+// NoHandler is a Handler that does not print anything
+var NoHandler noHandler
+
+// SavingHandler saves the events it receives.
+type SavingHandler struct {
+	mu sync.Mutex
+	v  []modelx.Measurement
+}
+
+// OnMeasurement implements modelx.Handler.OnMeasurement
+func (sh *SavingHandler) OnMeasurement(ev modelx.Measurement) {
+	sh.mu.Lock()
+	sh.v = append(sh.v, ev)
+	sh.mu.Unlock()
+}
+
+// Read extracts the saved events
+func (sh *SavingHandler) Read() []modelx.Measurement {
+	sh.mu.Lock()
+	v := sh.v
+	sh.v = nil
+	sh.mu.Unlock()
+	return v
+}

internal/engine/legacy/netx/handlers/handlers_test.go

@@ -0,0 +1,19 @@
+package handlers_test
+
+import (
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/handlers"
+	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/modelx"
+)
+
+func TestGood(t *testing.T) {
+	handlers.NoHandler.OnMeasurement(modelx.Measurement{})
+	handlers.StdoutHandler.OnMeasurement(modelx.Measurement{})
+	saver := handlers.SavingHandler{}
+	saver.OnMeasurement(modelx.Measurement{})
+	events := saver.Read()
+	if len(events) != 1 {
+		t.Fatal("invalid number of events")
+	}
+}