[forwardport] refactor(oonimkall): merge internal/task into oonimkall (#617) (#618)

This diff forward ports bc4b9f1ea89158bfa7b7a80ae59a90b43c784ed2 to `master`.

See https://github.com/ooni/probe/issues/1903

pkg/oonimkall/eventemitter.go

@@ -0,0 +1,40 @@
+package oonimkall
+
+// 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] {
+		return
+	}
+	ee.out <- &event{Key: key, Value: value}
+}