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/tcpconnect.go

@@ -1,71 +0,0 @@
-package webconnectivity
-
-import (
-	"context"
-	"sync"
-
-	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
-	"github.com/ooni/probe-cli/v3/internal/model"
-	"github.com/ooni/probe-cli/v3/internal/netxlite"
-)
-
-// CtrlTCPResult is the result of the TCP check performed by the test helper.
-type CtrlTCPResult = webconnectivity.ControlTCPConnectResult
-
-// TCPResultPair contains the endpoint and the corresponding result.
-type TCPResultPair struct {
-	Endpoint string
-	Result   CtrlTCPResult
-}
-
-// TCPConfig configures the TCP connect check.
-type TCPConfig struct {
-	Endpoint  string
-	NewDialer func() model.Dialer
-	Out       chan TCPResultPair
-	Wg        *sync.WaitGroup
-}
-
-// TCPDo performs the TCP check.
-func TCPDo(ctx context.Context, config *TCPConfig) {
-	defer config.Wg.Done()
-	dialer := config.NewDialer()
-	defer dialer.CloseIdleConnections()
-	conn, err := dialer.DialContext(ctx, "tcp", config.Endpoint)
-	if conn != nil {
-		conn.Close()
-	}
-	config.Out <- TCPResultPair{
-		Endpoint: config.Endpoint,
-		Result: CtrlTCPResult{
-			Failure: tcpMapFailure(newfailure(err)),
-			Status:  err == nil,
-		},
-	}
-}
-
-// tcpMapFailure attempts to map netxlite failures to the strings
-// used by the original OONI test helper.
-//
-// See https://github.com/ooni/backend/blob/6ec4fda5b18/oonib/testhelpers/http_helpers.py#L392
-func tcpMapFailure(failure *string) *string {
-	switch failure {
-	case nil:
-		return nil
-	default:
-		switch *failure {
-		case netxlite.FailureGenericTimeoutError:
-			return failure // already using the same name
-		case netxlite.FailureConnectionRefused:
-			s := "connection_refused_error"
-			return &s
-		default:
-			// The definition of this error according to Twisted is
-			// "something went wrong when connecting". Because we are
-			// indeed basically just connecting here, it seems safe
-			// to map any other error to "connect_error" here.
-			s := "connect_error"
-			return &s
-		}
-	}
-}