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/dnsoverudp.go

@@ -0,0 +1,64 @@
+package resolver
+
+import (
+	"context"
+	"net"
+	"time"
+)
+
+// Dialer is the network dialer interface assumed by this package.
+type Dialer interface {
+	DialContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// DNSOverUDP is a DNS over UDP RoundTripper.
+type DNSOverUDP struct {
+	dialer  Dialer
+	address string
+}
+
+// NewDNSOverUDP creates a DNSOverUDP instance.
+func NewDNSOverUDP(dialer Dialer, address string) DNSOverUDP {
+	return DNSOverUDP{dialer: dialer, address: address}
+}
+
+// RoundTrip implements RoundTripper.RoundTrip.
+func (t DNSOverUDP) RoundTrip(ctx context.Context, query []byte) ([]byte, error) {
+	conn, err := t.dialer.DialContext(ctx, "udp", t.address)
+	if err != nil {
+		return nil, err
+	}
+	defer conn.Close()
+	// Use five seconds timeout like Bionic does. See
+	// https://labs.ripe.net/Members/baptiste_jonglez_1/persistent-dns-connections-for-reliability-and-performance
+	if err = conn.SetDeadline(time.Now().Add(5 * time.Second)); err != nil {
+		return nil, err
+	}
+	if _, err = conn.Write(query); err != nil {
+		return nil, err
+	}
+	reply := make([]byte, 1<<17)
+	var n int
+	n, err = conn.Read(reply)
+	if err != nil {
+		return nil, err
+	}
+	return reply[:n], nil
+}
+
+// RequiresPadding returns false for UDP according to RFC8467
+func (t DNSOverUDP) RequiresPadding() bool {
+	return false
+}
+
+// Network returns the transport network (e.g., doh, dot)
+func (t DNSOverUDP) Network() string {
+	return "udp"
+}
+
+// Address returns the upstream server address.
+func (t DNSOverUDP) Address() string {
+	return t.address
+}
+
+var _ RoundTripper = DNSOverUDP{}