guix: Separate the package name and version with "@", not "-".

guix: Separate the package name and version with "@", not "-".

* guix/packages.scm (package-full-name): By default, use "@" to separate
  the package name and package version.  Add an optional delimiter
  argument so that there is still a way to explicitly use a different
  delimiter.
* gnu/packages/commencement.scm (gcc-boot0) <unpack-gmp&co>: Adjust
  accordingly.
* tests/graph.scm: Adjust accordingly.
* tests/profiles.scm: Adjust accordingly.
* NEWS: Mention the change.

Fixes: <https://bugs.gnu.org/31088>.
Reported by Pierre Neidhardt <ambrevar@gmail.com>.