Remove the default command

### What needs to get done

Remove support for the dispatcher's default command.

### Why it needs to get done

A default command was added to craft-cli to maintain compatibility with Snapcraft. Default commands aren't a good UX practice and no other craft app supports them.

Snapcraft 9 dropped support for the default command, but craft-cli still needs to provide a helpful error message:
```
Running snapcraft without a command isn't supported.
Use 'snapcraft pack' instead.
```

Craft-cli can fully drop support when Snapcraft 10 (core28) is released, as this version of Snapcraft will do what the other apps do (show the help text).