Revert "move pkg/search contents to pkg/search/zoekt (#3012)"
Created by: ijt
This reverts commit 0400f0988649cfb1f6db74d6d7ca5996148b6d54.
According to @keegancsmith: """ FYI pkg/search wasn't zoekt specific. It was the new backend code for hierarchical search. It implemented a Searcher for both our searcher backend and zoekt. """
Test plan: The code is just being moved. There is nothing new to test.