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/multierror/multierror.go

@@ -0,0 +1,66 @@
+// Package multierror contains code to manage multiple errors.
+package multierror
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// Union is the logical union of several errors. The Union will
+// appear to be the Root error, except that it will actually
+// be possible to look deeper and see specific sub errors that
+// occurred using errors.As and errors.Is.
+type Union struct {
+	Children []error
+	Root     error
+}
+
+// New creates a new Union error instance.
+func New(root error) *Union {
+	return &Union{Root: root}
+}
+
+// Unwrap returns the Root error of the Union error.
+func (err Union) Unwrap() error {
+	return err.Root
+}
+
+// Add adds the specified child error to the Union error.
+func (err *Union) Add(child error) {
+	err.Children = append(err.Children, child)
+}
+
+// AddWithPrefix adds the specified child error to the Union error
+// with the specified prefix before the child error.
+func (err *Union) AddWithPrefix(prefix string, child error) {
+	err.Add(fmt.Errorf("%s: %w", prefix, child))
+}
+
+// Is returns whether the Union error contains at least one child
+// error that is exactly the specified target error.
+func (err Union) Is(target error) bool {
+	if errors.Is(err.Root, target) {
+		return true
+	}
+	for _, c := range err.Children {
+		if errors.Is(c, target) {
+			return true
+		}
+	}
+	return false
+}
+
+// Error returns a string representation of the Union error.
+func (err Union) Error() string {
+	var sb strings.Builder
+	sb.WriteString(err.Root.Error())
+	sb.WriteString(": [")
+	for _, c := range err.Children {
+		sb.WriteString(" ")
+		sb.WriteString(c.Error())
+		sb.WriteString(";")
+	}
+	sb.WriteString("]")
+	return sb.String()
+}