searcher: factor client out of graphlbackend

Created by: keegancsmith

This doesn't improve the client or change it at all. It just cuts and pastes it into its own package.

We now will "copy" the data. Previously we intentionally unmarshalled straight into the resolvers to be more efficient. However, we never do large searches with searcher anymore. Additionally Zoekt does large scale searches and also does a copy.

This change moves us towards a potentially more general interface that all search clients could implement.