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_test.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+)
+
+func Test_tcpMapFailure(t *testing.T) {
+	tests := []struct {
+		name    string
+		failure *string
+		want    *string
+	}{{
+		name:    "nil",
+		failure: nil,
+		want:    nil,
+	}, {
+		name:    "timeout",
+		failure: stringPointerForString(netxlite.FailureGenericTimeoutError),
+		want:    stringPointerForString(netxlite.FailureGenericTimeoutError),
+	}, {
+		name:    "connection refused",
+		failure: stringPointerForString(netxlite.FailureConnectionRefused),
+		want:    stringPointerForString("connection_refused_error"),
+	}, {
+		name:    "anything else",
+		failure: stringPointerForString(netxlite.FailureEOFError),
+		want:    stringPointerForString("connect_error"),
+	}}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tcpMapFailure(tt.failure)
+			if diff := cmp.Diff(tt.want, got); diff != "" {
+				t.Fatal(diff)
+			}
+		})
+	}
+}