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/internal/webconnectivity/measure.go

@@ -1,92 +0,0 @@
-package webconnectivity
-
-import (
-	"context"
-	"net"
-	"net/url"
-	"sync"
-
-	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
-	"github.com/ooni/probe-cli/v3/internal/model"
-)
-
-type (
-	// CtrlRequest is the request sent to the test helper
-	CtrlRequest = webconnectivity.ControlRequest
-
-	// CtrlResponse is the response from the test helper
-	CtrlResponse = webconnectivity.ControlResponse
-)
-
-// MeasureConfig contains configuration for Measure.
-type MeasureConfig struct {
-	MaxAcceptableBody int64
-	NewClient         func() model.HTTPClient
-	NewDialer         func() model.Dialer
-	NewResolver       func() model.Resolver
-}
-
-// Measure performs the measurement described by the request and
-// returns the corresponding response or an error.
-func Measure(ctx context.Context, config MeasureConfig, creq *CtrlRequest) (*CtrlResponse, error) {
-	// parse input for correctness
-	URL, err := url.Parse(creq.HTTPRequest)
-	if err != nil {
-		return nil, err
-	}
-	// dns: start
-	wg := new(sync.WaitGroup)
-	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
-}