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/measure.go

@@ -0,0 +1,93 @@
+package main
+
+//
+// Top-level measurement algorithm
+//
+
+import (
+	"context"
+	"net"
+	"net/url"
+	"sync"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
+)
+
+type (
+	// ctrlRequest is the request sent to the test helper
+	ctrlRequest = webconnectivity.ControlRequest
+
+	// ctrlResponse is the response from the test helper
+	ctrlResponse = webconnectivity.ControlResponse
+)
+
+// measure performs the measurement described by the request and
+// returns the corresponding response or an error.
+func measure(ctx context.Context, config *handler, creq *ctrlRequest) (*ctrlResponse, error) {
+	// parse input for correctness
+	URL, err := url.Parse(creq.HTTPRequest)
+	if err != nil {
+		return nil, err
+	}
+	wg := &sync.WaitGroup{}
+
+	// dns: start
+	dnsch := make(chan ctrlDNSResult, 1)
+	if net.ParseIP(URL.Hostname()) == nil {
+		wg.Add(1)
+		go dnsDo(ctx, &dnsConfig{
+			Domain:      URL.Hostname(),
+			NewResolver: config.NewResolver,
+			Out:         dnsch,
+			Wg:          wg,
+		})
+	}
+
+	// tcpconnect: start
+	tcpconnch := make(chan tcpResultPair, len(creq.TCPConnect))
+	for _, endpoint := range creq.TCPConnect {
+		wg.Add(1)
+		go tcpDo(ctx, &tcpConfig{
+			Endpoint:  endpoint,
+			NewDialer: config.NewDialer,
+			Out:       tcpconnch,
+			Wg:        wg,
+		})
+	}
+
+	// http: start
+	httpch := make(chan ctrlHTTPResponse, 1)
+	wg.Add(1)
+	go httpDo(ctx, &httpConfig{
+		Headers:           creq.HTTPRequestHeaders,
+		MaxAcceptableBody: config.MaxAcceptableBody,
+		NewClient:         config.NewClient,
+		Out:               httpch,
+		URL:               creq.HTTPRequest,
+		Wg:                wg,
+	})
+
+	// wait for measurement steps to complete
+	wg.Wait()
+
+	// assemble response
+	cresp := new(ctrlResponse)
+	select {
+	case cresp.DNS = <-dnsch:
+	default:
+		// we need to emit a non-nil Addrs to match exactly
+		// the behavior of the legacy TH
+		cresp.DNS = ctrlDNSResult{
+			Failure: nil,
+			Addrs:   []string{},
+		}
+	}
+	cresp.HTTPRequest = <-httpch
+	cresp.TCPConnect = make(map[string]ctrlTCPResult)
+	for len(cresp.TCPConnect) < len(creq.TCPConnect) {
+		tcpconn := <-tcpconnch
+		cresp.TCPConnect[tcpconn.Endpoint] = tcpconn.Result
+	}
+
+	return cresp, nil
+}