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/cmd/oohelperd/oohelperd.go

@@ -0,0 +1,67 @@
+// Command oohelperd contains the Web Connectivity test helper.
+package main
+
+import (
+	"context"
+	"flag"
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/cmd/oohelperd/internal"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+)
+
+const maxAcceptableBody = 1 << 24
+
+var (
+	dialer    netx.Dialer
+	endpoint  = flag.String("endpoint", ":8080", "Endpoint where to listen")
+	httpx     *http.Client
+	resolver  netx.Resolver
+	srvcancel context.CancelFunc
+	srvctx    context.Context
+	srvwg     = new(sync.WaitGroup)
+)
+
+func init() {
+	srvctx, srvcancel = context.WithCancel(context.Background())
+	dialer = netx.NewDialer(netx.Config{Logger: log.Log})
+	txp := netx.NewHTTPTransport(netx.Config{Logger: log.Log})
+	httpx = &http.Client{Transport: txp}
+	resolver = netx.NewResolver(netx.Config{Logger: log.Log})
+}
+
+func shutdown(srv *http.Server) {
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+	srv.Shutdown(ctx)
+}
+
+func main() {
+	logmap := map[bool]log.Level{
+		true:  log.DebugLevel,
+		false: log.InfoLevel,
+	}
+	debug := flag.Bool("debug", false, "Toggle debug mode")
+	flag.Parse()
+	log.SetLevel(logmap[*debug])
+	testableMain()
+}
+
+func testableMain() {
+	mux := http.NewServeMux()
+	mux.Handle("/", internal.Handler{
+		Client:            httpx,
+		Dialer:            dialer,
+		MaxAcceptableBody: maxAcceptableBody,
+		Resolver:          resolver,
+	})
+	srv := &http.Server{Addr: *endpoint, Handler: mux}
+	srvwg.Add(1)
+	go srv.ListenAndServe()
+	<-srvctx.Done()
+	shutdown(srv)
+	srvwg.Done()
+}