utils: Use '@' for separating package names and version numbers.

This provides the ability to use numbers in package names.

Fixes <http://bugs.gnu.org/19219>.

* guix/utils.scm (package-name->name+version): New procedure.
* gnu/packages.scm (%find-package): Add a FALLBACK? keyword argument.
Use the previous method when no package is found.
(specification->package+output, specification->package): Adapt
documentation to new syntax.
* doc/guix.texi (Invoking guix package, Invoking guix import): Likewise.
* guix/ui.scm (package-specification->name+version+output): Likewise.
* guix/scripts/import/hackage.scm (show-help): Likewise.
* tests/guix-build.sh: Adapt to new syntax.
* tests/guix-lint.sh: Likewise.
* tests/guix-package.sh: Likewise.
* tests/ui.scm ("package-specification->name+version+output"): Likewise.
* tests/utils.scm ("package-name->name+version"): Likewise.
* NEWS: Mention new syntax.