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/internal/randx/randx.go

@@ -0,0 +1,50 @@
+// Package randx contains math/rand extensions
+package randx
+
+import (
+	"math/rand"
+	"time"
+	"unicode"
+)
+
+const (
+	uppercase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+	lowercase = "abcdefghijklmnopqrstuvwxyz"
+	letters   = uppercase + lowercase
+)
+
+func lettersWithString(n int, letterBytes string) string {
+	// See https://stackoverflow.com/questions/22892120
+	rnd := rand.New(rand.NewSource(time.Now().UnixNano()))
+	b := make([]byte, n)
+	for i := range b {
+		b[i] = letterBytes[rnd.Intn(len(letterBytes))]
+	}
+	return string(b)
+}
+
+// Letters return a string composed of random letters
+func Letters(n int) string {
+	return lettersWithString(n, letters)
+}
+
+// LettersUppercase return a string composed of random uppercase letters
+func LettersUppercase(n int) string {
+	return lettersWithString(n, uppercase)
+}
+
+// ChangeCapitalization returns a new string where the capitalization
+// of each character is changed at random.
+func ChangeCapitalization(source string) (dest string) {
+	rnd := rand.New(rand.NewSource(time.Now().UnixNano()))
+	for _, chr := range source {
+		if unicode.IsLower(chr) && rnd.Float64() <= 0.5 {
+			dest += string(unicode.ToUpper(chr))
+		} else if unicode.IsUpper(chr) && rnd.Float64() <= 0.5 {
+			dest += string(unicode.ToLower(chr))
+		} else {
+			dest += string(chr)
+		}
+	}
+	return
+}