Implement morelike: for more like this.

Also some (more) code reorganization that makes sense in light of the
new search type.

Bug: 53474

Change-Id: Ib1e27dfcf980efa52414385c0b9264c1392bd2a1
Tests for morelike:.

Bug: 53474
Change-Id: Id1c606bebdbc5211dbe9f622861ddc36f37c85d6
Merge "Implement morelike: for more like this."
Merge "Tests for morelike:."