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/cmd/jafar/resolver/resolver.go

@@ -0,0 +1,130 @@
+// Package resolver contains a censoring DNS resolver. Most queries are
+// answered without censorship, but selected queries could either be
+// discarded or replied to with a bogon or NXDOMAIN answer.
+package resolver
+
+import (
+	"context"
+	"net"
+	"strings"
+
+	"github.com/miekg/dns"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+)
+
+// CensoringResolver is a censoring resolver.
+type CensoringResolver struct {
+	blocked    []string
+	hijacked   []string
+	ignored    []string
+	lookupHost func(ctx context.Context, host string) ([]string, error)
+}
+
+// NewCensoringResolver creates a new CensoringResolver instance using
+// the specified list of keywords to censor. blocked is the list of
+// keywords that trigger NXDOMAIN if they appear in a query. hijacked
+// is similar but redirects to 127.0.0.1, where the transparent HTTP
+// and TLS proxies will pick them up. dnsNetwork and dnsAddress are the
+// settings to configure the upstream, non censored DNS.
+func NewCensoringResolver(
+	blocked, hijacked, ignored []string, uncensored netx.Resolver,
+) *CensoringResolver {
+	return &CensoringResolver{
+		blocked:    blocked,
+		hijacked:   hijacked,
+		ignored:    ignored,
+		lookupHost: uncensored.LookupHost,
+	}
+}
+
+func (r *CensoringResolver) roundtrip(rw dns.ResponseWriter, req *dns.Msg) {
+	name := req.Question[0].Name
+	addrs, err := r.lookupHost(context.Background(), name)
+	var ips []net.IP
+	if err == nil {
+		for _, addr := range addrs {
+			if ip := net.ParseIP(addr); ip != nil {
+				ips = append(ips, ip)
+			}
+		}
+	}
+	r.reply(rw, req, ips)
+}
+
+func (r *CensoringResolver) reply(
+	rw dns.ResponseWriter, req *dns.Msg, ips []net.IP,
+) {
+	m := new(dns.Msg)
+	m.Compress = true
+	m.MsgHdr.RecursionAvailable = true
+	m.SetReply(req)
+	for _, ip := range ips {
+		ipv6 := strings.Contains(ip.String(), ":")
+		if !ipv6 && req.Question[0].Qtype == dns.TypeA {
+			m.Answer = append(m.Answer, &dns.A{
+				Hdr: dns.RR_Header{
+					Name:   req.Question[0].Name,
+					Rrtype: dns.TypeA,
+					Class:  dns.ClassINET,
+					Ttl:    0,
+				},
+				A: ip,
+			})
+		}
+	}
+	if m.Answer == nil {
+		m.SetRcode(req, dns.RcodeNameError)
+	}
+	rw.WriteMsg(m)
+}
+
+func (r *CensoringResolver) failure(rw dns.ResponseWriter, req *dns.Msg) {
+	m := new(dns.Msg)
+	m.Compress = true
+	m.MsgHdr.RecursionAvailable = true
+	m.SetRcode(req, dns.RcodeServerFailure)
+	rw.WriteMsg(m)
+}
+
+// ServeDNS serves a DNS request
+func (r *CensoringResolver) ServeDNS(rw dns.ResponseWriter, req *dns.Msg) {
+	if len(req.Question) < 1 {
+		r.failure(rw, req)
+		return
+	}
+	name := req.Question[0].Name
+	for _, pattern := range r.blocked {
+		if strings.Contains(name, pattern) {
+			r.reply(rw, req, nil)
+			return
+		}
+	}
+	for _, pattern := range r.hijacked {
+		if strings.Contains(name, pattern) {
+			r.reply(rw, req, []net.IP{net.IPv4(127, 0, 0, 1)})
+			return
+		}
+	}
+	for _, pattern := range r.ignored {
+		if strings.Contains(name, pattern) {
+			return
+		}
+	}
+	r.roundtrip(rw, req)
+}
+
+// Start starts the DNS resolver
+func (r *CensoringResolver) Start(address string) (*dns.Server, error) {
+	packetconn, err := net.ListenPacket("udp", address)
+	if err != nil {
+		return nil, err
+	}
+	server := &dns.Server{
+		Addr:       address,
+		Handler:    r,
+		Net:        "udp",
+		PacketConn: packetconn,
+	}
+	go server.ActivateAndServe()
+	return server, nil
+}