Exodus does not support bundle migrations

I initially wanted to implement the removal of doctype from bundles as an exodus, as I was already doing with charms. But exoduses don't support bundles, only charms, because they accept CharmSource as their source and target.

The workaround is to use a non-exodus migration. If both bundles and charms need to be upgraded, the charm upgrade can be an exodus, while the bundle upgrade should be the next migration. (If it is migrated as part of the exodus, there is a risk of breaking running code.)

This works because there aren't many bundles at the moment, so exoduses are not yet required. This approach will need to be reconsidered as bundle quantities grow.