As per section '16.4.2 Package Naming' in the manual, use hyphens
instead of underscores in package names.
* guix/lint.scm (check-name): Check whether the package name contains
underscores.
* tests/lint.scm ("name: use underscore in package name"): New test.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
