chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/experiment/webconnectivity/dnsanalysis.go

@@ -0,0 +1,99 @@
+package webconnectivity
+
+import (
+	"net"
+	"net/url"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/errorx"
+)
+
+// DNSAnalysisResult contains the results of analysing comparing
+// the measurement and the control DNS results.
+type DNSAnalysisResult struct {
+	DNSConsistency *string `json:"dns_consistency"`
+}
+
+// DNSNameError is the error returned by the control on NXDOMAIN
+const DNSNameError = "dns_name_error"
+
+var (
+	// DNSConsistent indicates that the measurement and the
+	// control have consistent DNS results.
+	DNSConsistent = "consistent"
+
+	// DNSInconsistent indicates that the measurement and the
+	// control have inconsistent DNS results.
+	DNSInconsistent = "inconsistent"
+)
+
+// DNSAnalysis compares the measurement and the control DNS results. This
+// implementation is a simplified version of the implementation of the same
+// check implemented in Measurement Kit v0.10.11.
+func DNSAnalysis(URL *url.URL, measurement DNSLookupResult,
+	control ControlResponse) (out DNSAnalysisResult) {
+	// 0. start assuming it's not consistent
+	out.DNSConsistency = &DNSInconsistent
+	// 1. flip to consistent if we're targeting an IP address because the
+	// control will actually return dns_name_error in this case.
+	if net.ParseIP(URL.Hostname()) != nil {
+		out.DNSConsistency = &DNSConsistent
+		return
+	}
+	// 2. flip to consistent if the failures are compatible
+	if measurement.Failure != nil && control.DNS.Failure != nil {
+		switch *control.DNS.Failure {
+		case DNSNameError: // the control returns this on NXDOMAIN error
+			switch *measurement.Failure {
+			case errorx.FailureDNSNXDOMAINError:
+				out.DNSConsistency = &DNSConsistent
+			}
+		}
+		return
+	}
+	// 3. flip to consistent if measurement and control returned IP addresses
+	// that belong to the same Autonomous System(s).
+	//
+	// This specific check is present in MK's implementation.
+	//
+	// Note that this covers also the cases where the measurement contains only
+	// bogons while the control does not contain bogons.
+	//
+	// Note that this also covers the cases where results are equal.
+	const (
+		inMeasurement = 1 << 0
+		inControl     = 1 << 1
+		inBoth        = inMeasurement | inControl
+	)
+	asnmap := make(map[int64]int)
+	for _, asn := range measurement.Addrs {
+		asnmap[asn] |= inMeasurement
+	}
+	for _, asn := range control.DNS.ASNs {
+		asnmap[asn] |= inControl
+	}
+	for key, value := range asnmap {
+		// zero means that ASN lookup failed
+		if key != 0 && (value&inBoth) == inBoth {
+			out.DNSConsistency = &DNSConsistent
+			return
+		}
+	}
+	// 4. when ASN lookup failed (unlikely), check whether
+	// there is overlap in the returned IP addresses
+	ipmap := make(map[string]int)
+	for ip := range measurement.Addrs {
+		ipmap[ip] |= inMeasurement
+	}
+	for _, ip := range control.DNS.Addrs {
+		ipmap[ip] |= inControl
+	}
+	for key, value := range ipmap {
+		// just in case an empty string slipped through
+		if key != "" && (value&inBoth) == inBoth {
+			out.DNSConsistency = &DNSConsistent
+			return
+		}
+	}
+	// 5. conclude that measurement and control are inconsistent
+	return
+}