Add a :defaults "macro" to the :files spec.

This allows users to add files to the default file list without overwriting it. For instance:
```
(example :fetcher example :url "http://example.com/"
         :files (:defaults "an.example"))
```
This would fetch Emacs Lisp (except tests) and Texinfo files, just like the defaults, but unlike the defaults, the file `an.example` would be included too.

`:defaults` is only recognized when it's the first element of `:files`.

Fixes #2275.