wasi-sdk: add default provider

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>

etc/spack/defaults/packages.yaml

@@ -61,6 +61,7 @@ packages:
       tbb: [intel-tbb]
       unwind: [libunwind]
       uuid: [util-linux-uuid, libuuid]
+      wasi-sdk: [wasi-sdk-prebuilt]
       xxd: [xxd-standalone, vim]
       yacc: [bison, byacc]
       ziglang: [zig]