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/emitter.go

@@ -0,0 +1,89 @@
+package resolver
+
+import (
+	"context"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/dialid"
+	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/modelx"
+	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/transactionid"
+)
+
+// EmitterTransport is a RoundTripper that emits events when they occur.
+type EmitterTransport struct {
+	RoundTripper
+}
+
+// RoundTrip implements RoundTripper.RoundTrip
+func (txp EmitterTransport) RoundTrip(ctx context.Context, querydata []byte) ([]byte, error) {
+	root := modelx.ContextMeasurementRootOrDefault(ctx)
+	root.Handler.OnMeasurement(modelx.Measurement{
+		DNSQuery: &modelx.DNSQueryEvent{
+			Data:                   querydata,
+			DialID:                 dialid.ContextDialID(ctx),
+			DurationSinceBeginning: time.Now().Sub(root.Beginning),
+		},
+	})
+	replydata, err := txp.RoundTripper.RoundTrip(ctx, querydata)
+	if err != nil {
+		return nil, err
+	}
+	root.Handler.OnMeasurement(modelx.Measurement{
+		DNSReply: &modelx.DNSReplyEvent{
+			Data:                   replydata,
+			DialID:                 dialid.ContextDialID(ctx),
+			DurationSinceBeginning: time.Now().Sub(root.Beginning),
+		},
+	})
+	return replydata, nil
+}
+
+// EmitterResolver is a resolver that emits events
+type EmitterResolver struct {
+	Resolver
+}
+
+// LookupHost returns the IP addresses of a host
+func (r EmitterResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	var (
+		network string
+		address string
+	)
+	type queryableResolver interface {
+		Transport() RoundTripper
+	}
+	if qr, ok := r.Resolver.(queryableResolver); ok {
+		txp := qr.Transport()
+		network, address = txp.Network(), txp.Address()
+	}
+	dialID := dialid.ContextDialID(ctx)
+	txID := transactionid.ContextTransactionID(ctx)
+	root := modelx.ContextMeasurementRootOrDefault(ctx)
+	root.Handler.OnMeasurement(modelx.Measurement{
+		ResolveStart: &modelx.ResolveStartEvent{
+			DialID:                 dialID,
+			DurationSinceBeginning: time.Now().Sub(root.Beginning),
+			Hostname:               hostname,
+			TransactionID:          txID,
+			TransportAddress:       address,
+			TransportNetwork:       network,
+		},
+	})
+	addrs, err := r.Resolver.LookupHost(ctx, hostname)
+	root.Handler.OnMeasurement(modelx.Measurement{
+		ResolveDone: &modelx.ResolveDoneEvent{
+			Addresses:              addrs,
+			DialID:                 dialID,
+			DurationSinceBeginning: time.Now().Sub(root.Beginning),
+			Error:                  err,
+			Hostname:               hostname,
+			TransactionID:          txID,
+			TransportAddress:       address,
+			TransportNetwork:       network,
+		},
+	})
+	return addrs, err
+}
+
+var _ RoundTripper = EmitterTransport{}
+var _ Resolver = EmitterResolver{}