`base: core22` `default-provider:` allows channels, but SnapD doesn't

Snapcraft doesn't let you build a snap with a `default-provider:` that isn't available (yet):

```yaml
plugs:
  graphics-core22:
    interface: content
    target: $SNAP/graphics
    default-provider: mesa-core22
```

```shell
$ snapcraft
Launching instance...
Error installing snap 'mesa-core22' from channel 'latest/stable'.
Failed to execute pack in instance.
Full execution log: '/home/michal/.local/state/snapcraft/log/snapcraft-20230317-103905.303296.log'
```

It will happily build, if you give it a channel:

```yaml
default-provider: mesa-core22/edge
```

But then SnapD can't install it:

```shell
$ snap install graphics-test-tools --channel 22/edge
error: cannot perform the following tasks:
- Ensure prerequisites for "graphics-test-tools" are available (cannot install prerequisite "mesa-core22/edge": invalid instance name: invalid snap name: "mesa-core22/edge")
```

Snapcraft should either allow non-existent `default-provider:`s, or at least align with SnapD. Documentation does not mention channel support, so that's likely by design:

https://snapcraft.io/docs/content-interface#heading--default