The Package definition seems to now support defining a flag named solution on a Datasource whose purpose is to instruct the Kibana UI that a custom UI should be displayed. See Endpoint definition here:

https://github.com/elastic/package-storage/blob/master/packages/endpoint/0.1.0/manifest.yml#L23

datasources:
  - name: endpoint
    title: Endpoint data source
    description: Interact with the endpoint.

    # This tells the UI that for configuration, it must link to a specific solution
    # Only solution packages can contain this field.
    solution: endpoint

The Kibana Ingest Manager creation and editing of Datasources should be refactored to now process based on this flag being set to a know value.

(see this PR comment for reference)