Using the example as the default contents of an array.

images-tmpl.yml

#@ load("@ytt:data", "data")

#@ def get_package_repository(repository_name, package_name):
#@   if repository_name == "":
#@    for repository in data.values.repositories:
#@      for package in data.values.repositories[repository].packages:
#@        if package.name == package_name:
#@          return data.values.repositories[repository]
#@        end
#@      end
#@    end
#@   else:
#@     return data.values.repositories[repository_name]
#@   end
#@ end

#@ package_repository = get_package_repository("mgmt-repo", "")
---
apiVersion: imgpkg.carvel.dev/v1alpha1
images: #@ package_repository
kind: ImagesLock

values.yaml

#@data/values-schema
---
repositories:
  mgmt-repo:
    #@schema/default [{}]
    packages:
    - name: kapp
      displayName: kappdn
      version: v1