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/fsx/fsx.go

@@ -0,0 +1,51 @@
+// Package fsx contains file system extension
+package fsx
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+)
+
+// File is a generic file. This interface is taken from the draft
+// iofs golang design. We'll use fs.File when available.
+type File interface {
+	Stat() (os.FileInfo, error)
+	Read([]byte) (int, error)
+	Close() error
+}
+
+// FS is a generic file system. Like File, it's adapted from
+// the draft iofs golang design document.
+type FS interface {
+	Open(name string) (File, error)
+}
+
+// Open is a wrapper for os.Open that ensures that we're opening a file.
+func Open(pathname string) (File, error) {
+	return OpenWithFS(filesystem{}, pathname)
+}
+
+// OpenWithFS is like Open but with explicit file system argument.
+func OpenWithFS(fs FS, pathname string) (File, error) {
+	file, err := fs.Open(pathname)
+	if err != nil {
+		return nil, err
+	}
+	info, err := file.Stat()
+	if err != nil {
+		file.Close()
+		return nil, err
+	}
+	if info.IsDir() {
+		file.Close()
+		return nil, fmt.Errorf("input path points to a directory: %w", syscall.EISDIR)
+	}
+	return file, nil
+}
+
+type filesystem struct{}
+
+func (filesystem) Open(pathname string) (File, error) {
+	return os.Open(pathname)
+}