Remove is_fallback() from I*Form extension interfaces?

[seanh]

If a plugin wants to take over the default package type for example, it can return dataset? from its package_types() method. I don't see how there can be a package type that is not the default 'package' and is not returned by the package_types() method of a plugin, because the URLs for such a package type would not be mapped. Am I missing something or is is_fallback() not needed?

Same for IGroupForm and IOrganizationForm