indexer: Use CompositeObjId as object id type for ContentIndexer

Previously, content indexers were only using content sha1 as object identifier but this could lead to errors in production when attempting to fetch content data from an object storage where key is the sha256 checksum of the content (winery for instance).

So ensure to use a CompositeObjId dictionary as content object identifier to avoid these kind of issues when indexing content objects.

Should fix SWH-INDEXER-SG.