Add `wp-cli/scaffold-command` as a dependency

[Alain Schlesser]

Now that wp-cli/wp-cli is being restructured and only represents the framework, we don't have the circular dependency problem anymore and can be clearer about real dependencies.

The wp-cli/scaffold-package-command package should require wp-cli/scaffold-command as a dependency, as it might be pulled in outside of the regular bundle and should still work properly in such a case.