emacs: Use '@' to separate package names and version numbers.

This is a followup to commit 1b846da8.

* emacs/guix-base.el (guix-package-name-specification): Use "@" instead
of "-".
* emacs/guix-main.scm (name+version->full-name): Likewise.
(package-inputs-names): Use 'make-package-specification' instead of
'package-full-name'.
(full-name->name+version): Update the docstring.
* emacs/guix-ui-package.el (guix-packages-by-name): Likewise.