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/dnslookup.go

@@ -0,0 +1,59 @@
+package webconnectivity
+
+import (
+	"context"
+	"fmt"
+	"net/url"
+	"sort"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/urlgetter"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+// DNSLookupConfig contains settings for the DNS lookup.
+type DNSLookupConfig struct {
+	Session model.ExperimentSession
+	URL     *url.URL
+}
+
+// DNSLookupResult contains the result of the DNS lookup.
+type DNSLookupResult struct {
+	Addrs    map[string]int64
+	Failure  *string
+	TestKeys urlgetter.TestKeys
+}
+
+// DNSLookup performs the DNS lookup part of Web Connectivity.
+func DNSLookup(ctx context.Context, config DNSLookupConfig) (out DNSLookupResult) {
+	target := fmt.Sprintf("dnslookup://%s", config.URL.Hostname())
+	config.Session.Logger().Infof("%s...", target)
+	result, err := urlgetter.Getter{Session: config.Session, Target: target}.Get(ctx)
+	out.Addrs = make(map[string]int64)
+	for _, query := range result.Queries {
+		for _, answer := range query.Answers {
+			if answer.IPv4 != "" {
+				out.Addrs[answer.IPv4] = answer.ASN
+				continue
+			}
+			if answer.IPv6 != "" {
+				out.Addrs[answer.IPv6] = answer.ASN
+			}
+		}
+	}
+	config.Session.Logger().Infof("%s... %+v", target, err)
+	out.Failure = result.Failure
+	out.TestKeys = result
+	return
+}
+
+// Addresses returns the IP addresses in the DNSLookupResult
+func (r DNSLookupResult) Addresses() (out []string) {
+	out = []string{}
+	for addr := range r.Addrs {
+		out = append(out, addr)
+	}
+	sort.Slice(out, func(i, j int) bool {
+		return out[i] < out[j]
+	})
+	return
+}