Bad data used to build default importers map

[NightJar]

https://github.com/silverstripe/silverstripe-admin/blob/cd1271f86ba890a6d6378794bb58fd320751d29f/code/ModelAdmin.php#L616

The default importer fetch function supposes that the key for each managed model definition is the dataClass, but this is normlised to be in the dataClass key. The key from getManagedModels could be a URL Slug for which to load the managed model under, and not be valid class name at all. This results in an error when attempting to import data as the key is undefined when referenced in import()