Renamed index.js file to a service

refs https://github.com/TryGhost/Team/issues/958

- The module contains a service class and not an api index as index.js file should. This rename also fixes an ESLint warning around index.js file being too complicated.
- The serivice should ideally be extracted into the member repository in the future iteration

core/server/services/members/service.js

@@ -4,7 +4,7 @@ const MembersSSR = require('@tryghost/members-ssr');
 const db = require('../../data/db');
 const MembersConfigProvider = require('./config');
 const MembersCSVImporter = require('@tryghost/members-importer');
-const MembersStats = require('./stats');
+const MembersStats = require('./stats/members-stats');
 const createMembersApiInstance = require('./api');
 const createMembersSettingsInstance = require('./settings');
 const logging = require('@tryghost/logging');