search: factor out symbol search code

Created by: keegancsmith

I was trying to change this function and it was quite hard to understand. This was mostly due to the symbol search code that was added. This commit extracts the logic of how we create symbol resolvers from a zoekt.FileMatch. This also makes it clearer that baseURI and the commit resolver are only used by symbol search. We now avoid allocating those values in the more common text search.