Make sure pre migrators can also run as post migrator if the repository is 'new'

The use case is as follows:

A local upgrade is in progress and xml has been tranformed to yaml
The hst:channel nodes have not been migrated to the new location where they should be

In the above use case, we want after the configuration model has been applied (and the node /hst:hst/hst:channels exists), run the HST channel migrator with auto export enabled. If auto export is disabled, the repository startup can be short circuited with the mention that auto export must be enabled to transform the model.