LSIF: GraphQL enhancements

Created by: efritz

This PR improves the LSIF GraphQL API:

• removes LSIFDump and LSIFJob as these can just get a node directly by identifier
• moves LSIFDump under Repository resolver

This PR also changes code structure a bit:

• a8n and codeintel resolvers are no longer assigned to schemaResolver, but to a OptionalResolvers struct defined in the graphqlbackend package. This is necessary as the repositoryResolver (which is created directly in many places) also needs a reference to these optionally assigned resolvers (and there is no current way to pass them from the schema resolver as repository resolvers can be created independently).