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.