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/oonimkall/tasks/eventemitter.go

@@ -0,0 +1,40 @@
+package tasks
+
+// EventEmitter emits event on a channel
+type EventEmitter struct {
+	disabled map[string]bool
+	out      chan<- *Event
+}
+
+// NewEventEmitter creates a new Emitter
+func NewEventEmitter(disabledEvents []string, out chan<- *Event) *EventEmitter {
+	ee := &EventEmitter{out: out}
+	ee.disabled = make(map[string]bool)
+	for _, eventname := range disabledEvents {
+		ee.disabled[eventname] = true
+	}
+	return ee
+}
+
+// EmitFailureStartup emits the failureStartup event
+func (ee *EventEmitter) EmitFailureStartup(failure string) {
+	ee.EmitFailureGeneric(failureStartup, failure)
+}
+
+// EmitFailureGeneric emits a failure event
+func (ee *EventEmitter) EmitFailureGeneric(name, failure string) {
+	ee.Emit(name, EventFailure{Failure: failure})
+}
+
+// EmitStatusProgress emits the status.Progress event
+func (ee *EventEmitter) EmitStatusProgress(percentage float64, message string) {
+	ee.Emit(statusProgress, EventStatusProgress{Message: message, Percentage: percentage})
+}
+
+// Emit emits the specified event
+func (ee *EventEmitter) Emit(key string, value interface{}) {
+	if ee.disabled[key] == true {
+		return
+	}
+	ee.out <- &Event{Key: key, Value: value}
+}