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/date/date.service.js

@@ -1,7 +1,7 @@
 'use strict'
 
-const BaseService = require('../base')
 const { formatRelativeDate } = require('../../lib/text-formatters')
+const { BaseService } = require('..')
 
 const documentation = `
 <p>

services/date/date.tester.js

@@ -1,11 +1,13 @@
 'use strict'
 
 const Joi = require('joi')
-const ServiceTester = require('../service-tester')
+const { ServiceTester } = require('..')
 const { isRelativeFormattedDate } = require('../test-validators')
 
-const t = new ServiceTester({ id: 'date', title: 'Relative Date Tests' })
-module.exports = t
+const t = (module.exports = new ServiceTester({
+  id: 'date',
+  title: 'Relative Date Tests',
+}))
 
 t.create('Relative date')
   .get('/1540814400.json')