Implement DownloadDir for chroot

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Description

At the moment, chroot simply fails with DownloadDir is not implemented for amazon-chroot, which is an artifact from the past. https://github.com/hashicorp/packer-plugin-sdk/blob/main/chroot/communicator.go#L126

The implementation can be easily done with reversing UploadDir defined above in the same file:

func (c *Communicator) DownloadDir(src string, dst string, exclude []string) error {
	// If src ends with a trailing "/", copy from "src/." so that
	// directory contents (including hidden files) are copied, but the
	// directory "src" is omitted.  BSD does this automatically when
	// the source contains a trailing slash, but linux does not.
	if src[len(src)-1] == '/' {
		src = src + "."
	}

	// TODO: remove any file copied if it appears in `exclude`
	chrootSrc := filepath.Join(c.Chroot, src)
	log.Printf("Downloading directory '%s' to '%s'", chrootSrc, dst)

	cmd, err := c.CmdWrapper(fmt.Sprintf("cp -R '%s' %s", chrootSrc, dst))
	if err != nil {
					return err
	}

	stderr := new(bytes.Buffer)
	shell := common.ShellCommand(cmd)
	shell.Env = append(shell.Env, "LANG=C")
	shell.Env = append(shell.Env, os.Environ()...)
	shell.Stderr = stderr
	if err := shell.Run(); err == nil {
					return err
	}

	if strings.Contains(stderr.String(), "No such file") {
					// This just means that the directory was empty. Just ignore it.
					return nil
	}

	return err
}