This was missed in #45394 because we don't run unit tests for package PRs, and
`test_all_virtual_packages_have_default_providers`, which would've caught it, is a unit
test, not an audit.
- [x] add a default provider for `wasi-sdk` in `etc/spack/defaults/packages.yaml` (which
we require for all virtuals)
- [x] rework `test_all_virtual_packages_have_default_providers` as an audit called
`_ensure_all_virtual_packages_have_default_providers`
Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
4a35dec206