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/chanlogger.go

@@ -0,0 +1,87 @@
+package tasks
+
+import (
+	"fmt"
+)
+
+// ChanLogger is a logger targeting a channel
+type ChanLogger struct {
+	emitter    *EventEmitter
+	hasdebug   bool
+	hasinfo    bool
+	haswarning bool
+	out        chan<- *Event
+}
+
+// Debug implements Logger.Debug
+func (cl *ChanLogger) Debug(msg string) {
+	if cl.hasdebug {
+		cl.emitter.Emit("log", EventLog{
+			LogLevel: "DEBUG",
+			Message:  msg,
+		})
+	}
+}
+
+// Debugf implements Logger.Debugf
+func (cl *ChanLogger) Debugf(format string, v ...interface{}) {
+	if cl.hasdebug {
+		cl.Debug(fmt.Sprintf(format, v...))
+	}
+}
+
+// Info implements Logger.Info
+func (cl *ChanLogger) Info(msg string) {
+	if cl.hasinfo {
+		cl.emitter.Emit("log", EventLog{
+			LogLevel: "INFO",
+			Message:  msg,
+		})
+	}
+}
+
+// Infof implements Logger.Infof
+func (cl *ChanLogger) Infof(format string, v ...interface{}) {
+	if cl.hasinfo {
+		cl.Info(fmt.Sprintf(format, v...))
+	}
+}
+
+// Warn implements Logger.Warn
+func (cl *ChanLogger) Warn(msg string) {
+	if cl.haswarning {
+		cl.emitter.Emit("log", EventLog{
+			LogLevel: "WARNING",
+			Message:  msg,
+		})
+	}
+}
+
+// Warnf implements Logger.Warnf
+func (cl *ChanLogger) Warnf(format string, v ...interface{}) {
+	if cl.haswarning {
+		cl.Warn(fmt.Sprintf(format, v...))
+	}
+}
+
+// NewChanLogger creates a new ChanLogger instance.
+func NewChanLogger(emitter *EventEmitter, logLevel string,
+	out chan<- *Event) *ChanLogger {
+	cl := &ChanLogger{
+		emitter: emitter,
+		out:     out,
+	}
+	switch logLevel {
+	case "DEBUG", "DEBUG2":
+		cl.hasdebug = true
+		fallthrough
+	case "INFO":
+		cl.hasinfo = true
+		fallthrough
+	case "ERR", "WARNING":
+		fallthrough
+	default:
+		cl.haswarning = true
+	}
+	return cl
+}