logging(gitstart): migrate graphqlbackend.RepositoryResolver to use sourcegraph/log

[bobheadxi]

Problem statement

https://github.com/sourcegraph/sourcegraph/issues/36382

Success criteria

• Add a logger to schemaResolver, and initialize it with log.Scoped (or logtest where appropriate) wherever schemaResolver is created (unless an existing logger is easily available, in which case that should be used)
• Add a logger to RepositoryResolver, and let it be injected from NewRepositoryResolver. Use log.Scoped (or logtest where appropriate) to provide the logger at all usages of NewRepositoryResolver
• 3 log15 usages inside cmd/frontend/graphqlbackend/repository.go are replaced with calls to the appropriate logger

Time estimate

• Pull requests with ~450 lines changed should take 24 hours at maximum. Ping the reviewer in the spec pull request if time-consuming changes are required.

[gitstart-app]

Here is the GitStart Ticket for this issue: https://app.gitstart.com/clients/sourcegraph/tickets/SG-37398