search: move search code into a new package: /pkg/search

Created by: ijt

Lots of search-related code is currently tangled up in the graphql layer of the frontend, and also sequestered in cmd/frontend/internal/pkg/search. It seems more logical to have all the search-related code under one directory tree so we can see its code coverage at a glance and more easily navigate it. In addition, we would like to try out the possibility of consolidating all the search-related microservices into a monolith that can easily be tested using go test.