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/legacy/netx/connid/connid.go

@@ -0,0 +1,31 @@
+// Package connid contains code to generate the connectionID
+package connid
+
+import (
+	"net"
+	"strconv"
+	"strings"
+)
+
+// Compute computes the connectionID from the local socket address. The zero
+// value is conventionally returned to mean "unknown".
+func Compute(network, address string) int64 {
+	_, portstring, err := net.SplitHostPort(address)
+	if err != nil {
+		return 0
+	}
+	portnum, err := strconv.Atoi(portstring)
+	if err != nil {
+		return 0
+	}
+	if portnum < 0 || portnum > 65535 {
+		return 0
+	}
+	result := int64(portnum)
+	if strings.Contains(network, "udp") {
+		result *= -1
+	} else if !strings.Contains(network, "tcp") {
+		result = 0
+	}
+	return result
+}

internal/engine/legacy/netx/connid/connid_test.go

@@ -0,0 +1,80 @@
+package connid
+
+import "testing"
+
+func TestTCP(t *testing.T) {
+	num := Compute("tcp", "1.2.3.4:6789")
+	if num != 6789 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestTCP4(t *testing.T) {
+	num := Compute("tcp4", "130.192.91.211:34566")
+	if num != 34566 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestTCP6(t *testing.T) {
+	num := Compute("tcp4", "[::1]:4444")
+	if num != 4444 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestUDP(t *testing.T) {
+	num := Compute("udp", "1.2.3.4:6789")
+	if num != -6789 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestUDP4(t *testing.T) {
+	num := Compute("udp4", "130.192.91.211:34566")
+	if num != -34566 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestUDP6(t *testing.T) {
+	num := Compute("udp6", "[::1]:4444")
+	if num != -4444 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestInvalidAddress(t *testing.T) {
+	num := Compute("udp6", "[::1]")
+	if num != 0 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestInvalidPort(t *testing.T) {
+	num := Compute("udp6", "[::1]:antani")
+	if num != 0 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestNegativePort(t *testing.T) {
+	num := Compute("udp6", "[::1]:-1")
+	if num != 0 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestLargePort(t *testing.T) {
+	num := Compute("udp6", "[::1]:65536")
+	if num != 0 {
+		t.Fatal("unexpected result")
+	}
+}
+
+func TestInvalidNetwork(t *testing.T) {
+	num := Compute("unix", "[::1]:65531")
+	if num != 0 {
+		t.Fatal("unexpected result")
+	}
+}