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/model/logger.go

@@ -0,0 +1,40 @@
+package model
+
+// Logger defines the common interface that a logger should have. It is
+// out of the box compatible with `log.Log` in `apex/log`.
+type Logger interface {
+	// Debug emits a debug message.
+	Debug(msg string)
+
+	// Debugf formats and emits a debug message.
+	Debugf(format string, v ...interface{})
+
+	// Info emits an informational message.
+	Info(msg string)
+
+	// Infof format and emits an informational message.
+	Infof(format string, v ...interface{})
+
+	// Warn emits a warning message.
+	Warn(msg string)
+
+	// Warnf formats and emits a warning message.
+	Warnf(format string, v ...interface{})
+}
+
+// DiscardLogger is a logger that discards its input
+var DiscardLogger Logger = logDiscarder{}
+
+type logDiscarder struct{}
+
+func (logDiscarder) Debug(msg string) {}
+
+func (logDiscarder) Debugf(format string, v ...interface{}) {}
+
+func (logDiscarder) Info(msg string) {}
+
+func (logDiscarder) Infof(format string, v ...interface{}) {}
+
+func (logDiscarder) Warn(msg string) {}
+
+func (logDiscarder) Warnf(format string, v ...interface{}) {}