fix: use cds.entities(<namespace>) (#213)

... to get definitions not exposed by the current service (i.e., `this`)

Author: sjvans

test/resources/bookshop/srv/cat-service.js

@@ -2,7 +2,7 @@ const cds = require('@sap/cds')
 
 class CatalogService extends cds.ApplicationService { init(){
 
-  const { Books } = this.entities ('sap.capire.bookshop')
+  const { Books } = cds.entities ('sap.capire.bookshop')
 
   // Reduce stock of ordered books if available stock suffices
   this.on ('submitOrder', async req => {