use `use-package-as-one` for normalizing `:ensure-system-package`

use `use-package-as-one` for normalizing `:ensure-system-package`

This makes the preferred syntax consistent with other `use-package`
keywords. All of these are now valid:

```
(use-package format-all
  :ensure-system-package
  (prettier . "npm i -g prettier")
  (rufo . "gem install rufo"))

(use-package format-all
  :ensure-system-package
  ((prettier . "npm i -g prettier")
   (rufo . "gem install rufo")))

(use-package format-all
  :ensure-system-package
  (prettier . "npm i -g prettier"))
```