For BC reasons when introducing `IncrementalIndexer`, instances of \eZ\Publish\Core\Search\Common\Indexer were also supported.

We can drop it now and introduce new `IncrementalIndexer` contracts as base implementation is no longer required and we don't want 3rd party to extends parts of Core.

The background reason for doing this is detection of too early resolution of SiteAccess configuration. Without rebuilding the whole indexer and its factory it's not possible to make it lazy.