Let PHLEX_REGISTER_SOURCE plugins see the job's stage #957

Sources registered with PHLEX_REGISTER_SOURCE build their own provider_bundles, and each provider_bundle carries a stage string. The plugin has no way to learn the stage configured for the job. source_graph_proxy is built from graph_registration_bundle::stage, but it only exposes add_source. Also, unlike provide() (registration_api.hpp), the bundles a source returns don't get "CURRENT" replaced with the job's stage.

So in aegir every generator source block repeats the stage from the top-level config:

{
  stage: 'simulation',
  sources: {
    gun: { cpp: 'particle_gun_source', stage: 'simulation', ... },
  },
}

The plugin reads that key, and we reject "CURRENT" because it would end up as a literal stage name. When the two values differ, for example after phlex --stage foo, the generator products are labelled simulation and everything downstream is labelled foo.

Either of these would let us drop the duplicated key:

1. a stage() accessor on source_graph_proxy, or
2. replacing "CURRENT" in the stage of the bundles a source returns, the way provide() already does.

#923 fixed the same gap for PHLEX_REGISTER_PROVIDERS providers.