refactor(oohelperd): flatten package hierarchy (#834)

In https://github.com/ooni/probe-cli/pull/832's initial diff, I
mentioned it would be cool to flatten oohelperd's hier.

I'm doing this now, and just for the master branch.

This diff is mostly a mechanical refactoring with very light
and apparently rather safe manual changes.

internal/cmd/oohelperd/main.go

@@ -0,0 +1,72 @@
+// Command oohelperd implements 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/model"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+)
+
+const maxAcceptableBody = 1 << 24
+
+var (
+	endpoint  = flag.String("endpoint", ":8080", "Endpoint where to listen")
+	srvcancel context.CancelFunc
+	srvctx    context.Context
+	srvwg     = new(sync.WaitGroup)
+)
+
+func init() {
+	srvctx, srvcancel = context.WithCancel(context.Background())
+}
+
+func newResolver() model.Resolver {
+	// Implementation note: pin to a specific resolver so we don't depend upon the
+	// default resolver configured by the box. Also, use an encrypted transport thus
+	// we're less vulnerable to any policy implemented by the box's provider.
+	resolver := netxlite.NewParallelDNSOverHTTPSResolver(log.Log, "https://8.8.8.8/dns-query")
+	return resolver
+}
+
+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("/", &handler{
+		MaxAcceptableBody: maxAcceptableBody,
+		NewClient: func() model.HTTPClient {
+			return netxlite.NewHTTPClientWithResolver(log.Log, newResolver())
+		},
+		NewDialer: func() model.Dialer {
+			return netxlite.NewDialerWithResolver(log.Log, newResolver())
+		},
+		NewResolver: newResolver,
+	})
+	srv := &http.Server{Addr: *endpoint, Handler: mux}
+	srvwg.Add(1)
+	go srv.ListenAndServe()
+	<-srvctx.Done()
+	shutdown(srv)
+	srvwg.Done()
+}