fix: use ajv for schema ref resolving (#462)

* fix: use ajv for schema ref resolving

* add tests for #350 and #417

* Don't print root schema id

* fix: remove schema setting by $id and key at the same time

* fix: clone external schemas

* Disable ajv schema validation

* Check is external schema exist

Author: ivan-tymoshenko

ajv.js

@@ -7,7 +7,7 @@ const ajvFormats = require('ajv-formats')
 module.exports = buildAjv
 
 function buildAjv (options) {
-  const ajvInstance = new Ajv({ ...options, strictSchema: false, uriResolver: fastUri })
+  const ajvInstance = new Ajv({ ...options, strictSchema: false, validateSchema: false, uriResolver: fastUri })
   ajvFormats(ajvInstance)
 
   const validateDateTimeFormat = ajvFormats.get('date-time').validate