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/tcpconnect.go

@@ -0,0 +1,85 @@
+package main
+
+//
+// TCP connect measurements
+//
+
+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 is the endpoint we measured.
+	Endpoint string
+
+	// Result contains the results.
+	Result ctrlTCPResult
+}
+
+// tcpConfig configures the TCP connect check.
+type tcpConfig struct {
+	// Endpoint is the MANDATORY endpoint to connect to.
+	Endpoint string
+
+	// NewDialer is the MANDATORY factory for creating a new dialer.
+	NewDialer func() model.Dialer
+
+	// Out is the MANDATORY where we'll post the TCP measurement results.
+	Out chan tcpResultPair
+
+	// Wg is MANDATORY and is used to sync with the parent.
+	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
+		}
+	}
+}