Create shortcut for BaseService-related imports (#2809)

Continue to implement #2698:

- Add `core/base-service/index.js` (but hold off on moving the things it imports)
- Add shortcuts in `services/index.js` for Base*Service, errors, and deprecatedService. This file will be streamlined later to avoid cluttering it with rarely used bits.
- Apply consistent ordering of imports and use of `module.exports` in testers.
- Remove some renaming of imports.
- Remove obsolete tests here and there.

services/sourcegraph/sourcegraph.service.js

@@ -1,7 +1,7 @@
 'use strict'
 
 const Joi = require('joi')
-const BaseJsonService = require('../base-json')
+const { BaseJsonService } = require('..')
 
 const projectsCountRegex = /^\s[0-9]*(\.[0-9]k)?\sprojects$/
 const schema = Joi.object({