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/netx/resolver/decoder.go

@@ -0,0 +1,54 @@
+package resolver
+
+import (
+	"errors"
+
+	"github.com/miekg/dns"
+)
+
+// The Decoder decodes a DNS reply into A or AAAA entries. It will use the
+// provided qtype and only look for mathing entries. It will return error if
+// there are no entries for the requested qtype inside the reply.
+type Decoder interface {
+	Decode(qtype uint16, data []byte) ([]string, error)
+}
+
+// MiekgDecoder uses github.com/miekg/dns to implement the Decoder.
+type MiekgDecoder struct{}
+
+// Decode implements Decoder.Decode.
+func (d MiekgDecoder) Decode(qtype uint16, data []byte) ([]string, error) {
+	reply := new(dns.Msg)
+	if err := reply.Unpack(data); err != nil {
+		return nil, err
+	}
+	// TODO(bassosimone): map more errors to net.DNSError names
+	switch reply.Rcode {
+	case dns.RcodeSuccess:
+	case dns.RcodeNameError:
+		return nil, errors.New("ooniresolver: no such host")
+	default:
+		return nil, errors.New("ooniresolver: query failed")
+	}
+	var addrs []string
+	for _, answer := range reply.Answer {
+		switch qtype {
+		case dns.TypeA:
+			if rra, ok := answer.(*dns.A); ok {
+				ip := rra.A
+				addrs = append(addrs, ip.String())
+			}
+		case dns.TypeAAAA:
+			if rra, ok := answer.(*dns.AAAA); ok {
+				ip := rra.AAAA
+				addrs = append(addrs, ip.String())
+			}
+		}
+	}
+	if len(addrs) <= 0 {
+		return nil, errors.New("ooniresolver: no response returned")
+	}
+	return addrs, nil
+}
+
+var _ Decoder = MiekgDecoder{}