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/bundlephobia/bundlephobia.service.js

@@ -1,8 +1,8 @@
 'use strict'
 
 const prettyBytes = require('pretty-bytes')
-const LegacyService = require('../legacy-service')
 const { makeBadgeData: getBadgeData } = require('../../lib/badge-data')
+const LegacyService = require('../legacy-service')
 
 const keywords = ['node']
 

services/bundlephobia/bundlephobia.tester.js

@@ -1,8 +1,8 @@
 'use strict'
 
 const Joi = require('joi')
-const ServiceTester = require('../service-tester')
 const { isFileSize } = require('../test-validators')
+const { ServiceTester } = require('..')
 
 const t = (module.exports = new ServiceTester({
   id: 'bundlephobia',