fix: remove hard dependency on model-fragments (#468)

* fix: remove hard dependency on model-fragments

* fix: instanceof Fragment and FragmentArray is not an object

Co-authored-by: Gabriel Cousin <7649529+GabrielCousin@users.noreply.github.com>

addon/converter/fixture-converter.js

@@ -4,6 +4,20 @@ import { entries } from '../utils/helper-functions';
 import { typeOf } from '@ember/utils';
 import { getOwner } from '@ember/application';
 import { camelize } from '@ember/string';
+import require from 'require';
+
+let Fragment;
+let FragmentArray;
+try {
+  let MF = require('ember-data-model-fragments/fragment');
+  let MFA = require('ember-data-model-fragments/array/fragment');
+  Fragment = MF && MF.default;
+  FragmentArray = MFA && MFA.default;
+} catch (e) {
+  // create empty constructors
+  Fragment = function Fragment() {};
+  FragmentArray = function FragmentArray() {};
+}
 
 /**
  Base class for converting the base fixture that factory guy creates to