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/dns.go

@@ -0,0 +1,84 @@
+package main
+
+//
+// DNS 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"
+	"github.com/ooni/probe-cli/v3/internal/tracex"
+)
+
+// newfailure is a convenience shortcut to save typing
+var newfailure = tracex.NewFailure
+
+// ctrlDNSResult is the result of the DNS check performed by
+// the Web Connectivity test helper.
+type ctrlDNSResult = webconnectivity.ControlDNSResult
+
+// dnsConfig configures the DNS check.
+type dnsConfig struct {
+	// Domain is the MANDATORY domain to resolve.
+	Domain string
+
+	// NewResolver is the MANDATORY factory to create a new resolver.
+	NewResolver func() model.Resolver
+
+	// Out is the channel where we publish the results.
+	Out chan ctrlDNSResult
+
+	// Wg allows to synchronize with the parent.
+	Wg *sync.WaitGroup
+}
+
+// dnsDo performs the DNS check.
+func dnsDo(ctx context.Context, config *dnsConfig) {
+	defer config.Wg.Done()
+	reso := config.NewResolver()
+	defer reso.CloseIdleConnections()
+	addrs, err := reso.LookupHost(ctx, config.Domain)
+	if addrs == nil {
+		addrs = []string{} // fix: the old test helper did that
+	}
+	failure := dnsMapFailure(newfailure(err))
+	config.Out <- ctrlDNSResult{Failure: failure, Addrs: addrs}
+}
+
+// dnsMapFailure 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#L430
+func dnsMapFailure(failure *string) *string {
+	switch failure {
+	case nil:
+		return nil
+	default:
+		switch *failure {
+		case netxlite.FailureDNSNXDOMAINError:
+			// We have a name for this string because dnsanalysis.go is
+			// already checking for this specific error string.
+			s := webconnectivity.DNSNameError
+			return &s
+		case netxlite.FailureDNSNoAnswer:
+			// In this case the legacy TH would produce an empty
+			// reply that is not attached to any error.
+			//
+			// See https://github.com/ooni/probe/issues/1707#issuecomment-944322725
+			return nil
+		case netxlite.FailureDNSNonRecoverableFailure,
+			netxlite.FailureDNSRefusedError,
+			netxlite.FailureDNSServerMisbehaving,
+			netxlite.FailureDNSTemporaryFailure:
+			s := "dns_server_failure"
+			return &s
+		default:
+			s := "unknown_error"
+			return &s
+		}
+	}
+}