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.tester.js

@@ -1,9 +1,10 @@
 'use strict'
 
 const Joi = require('joi')
-const t = (module.exports = require('../create-service-tester')())
 const { withRegex } = require('../test-validators')
 
+const t = (module.exports = require('..').createServiceTester())
+
 // Matches API responses such as "0 projects", "1 projects", "182 projects", "14.0k projects".
 // There may be other cases not covered by this regex, but hopefully the tested projects won't vary much.
 const projectsCount = withRegex(/^[0-9]*(\.[0-9]k)?\sprojects$/)