Unify the get_layers calls across all implements of manifest schemas to ensure we have a common type returned

Also renames some methods to make it more clear what kind of information they return

data/model/oci/manifest.py

@@ -96,12 +96,6 @@ def _create_manifest(repository_id, manifest_interface_instance, storage):
         logger.exception('Could not load manifest labels for child manifest')
         return None
 
-      # NOTE: Content type restrictions in manifest lists ensure that the child manifests
-      # must be image manifests, as opposed to lists themselves. We put this check here to
-      # be extra careful in ensuring we don't create empty manifests. If this reality changes,
-      # should remove this check.
-      assert list(child_manifest.layers)
-
       # Get/create the child manifest in the database.
       child_manifest_info = get_or_create_manifest(repository_id, child_manifest, storage)
       if child_manifest_info is None: