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/github/github-last-commit.tester.js

@@ -3,7 +3,7 @@
 const Joi = require('joi')
 const { isFormattedDate } = require('../test-validators')
 
-const t = (module.exports = require('../create-service-tester')())
+const t = (module.exports = require('..').createServiceTester())
 
 t.create('last commit (recent)')
   .get('/eslint/eslint.json')