codeintel: Refactor resolvers package

Created by: efritz

De-spaghettify the code intel resolvers package by:

• splitting the graphql resolvers from the underlying logic
• aggressively testing the logic resolvers
• making the graphql layer a simple wrapper over the logic resolvers that simply handle validation, serialization, and auth

Add lots of mocks and tests (which is the majority of the line count here).