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/netx/resolver/logging.go

@@ -0,0 +1,30 @@
+package resolver
+
+import (
+	"context"
+	"time"
+)
+
+// Logger is the logger assumed by this package
+type Logger interface {
+	Debugf(format string, v ...interface{})
+	Debug(message string)
+}
+
+// LoggingResolver is a resolver that emits events
+type LoggingResolver struct {
+	Resolver
+	Logger Logger
+}
+
+// LookupHost returns the IP addresses of a host
+func (r LoggingResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	r.Logger.Debugf("resolve %s...", hostname)
+	start := time.Now()
+	addrs, err := r.Resolver.LookupHost(ctx, hostname)
+	stop := time.Now()
+	r.Logger.Debugf("resolve %s... (%+v, %+v) in %s", hostname, addrs, err, stop.Sub(start))
+	return addrs, err
+}
+
+var _ Resolver = LoggingResolver{}