When in a bundle, check for bundle_core and bundle_site existing modules

Right now the Core and Site assignment plugins will create new "core" and "site" packages to contain suggested config.

If there is already a bundle_core or bundle_site package exported within the current bundle namespace, it would be better to assign the core and site config to those modules as new items.

This would prevent the problem of selecting the "core" package, clicking Write and having it overwrite the existing bundle_core module. It also provides a nicer user experience where auto-detected items from the Core and Site assignment plugins show up as newly detected values.

To prevent problems with existing bundle_core and bundle_site modules that do not want this behavior, we should probably add the shortname to the config settings for Core and Site so other names can be used if desired.