indexer_storage: Publish indexer computation to journal topics

This allows going straight from loading an origin up to indexing in elasticsearch's index origin loaded.

Related to T2817 Related to T2590

Test Plan

$ doco -f docker-compose.yml -f docker-compose.search.yml -f docker-compose.override.yml up -d`
$ doco -f docker-compose.yml -f docker-compose.search.yml -f docker-compose.override.yml exec swh-loader swh loader run https://github.com/codemeta/codemeta
$ http://localhost:9200/_cat/indices?v

health status index  uuid                   pri rep docs.count docs.deleted store.size pri.store.size
yellow open   origin 44iUHZqWST-QoE5eOA39oQ   1   1         13            5     65.1kb         65.1kb
# There is elasticsearch "document" in the "origin" index

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Migrated from D4709 (view on Phabricator)