remove symbol resolver from file match and suggestions

Created by: ijsnow

This PR decouples the symbolResolver type from search code. It does so by creating a searchSymbolResult that contains the data needed for toSymbolResolver and only uses toSymbolResolver when leaving the search portion of the graphqlbackend.

Helps with https://github.com/sourcegraph/sourcegraph/issues/3566.

Test plan: rely on existing tests and added one test