Do not assume global variables are assigned to self.

`requirejs` is a global variable (provided by [loader.js
here](https://github.com/ember-cli/loader.js/blob/v4.7.0/lib/loader/loader.js#L1)).
The prior code made the assumption that all global variables are
assigned to `self`/`window`, but this is _only_ true in some
circumstances. For example, when the entire bundle is evaluated within
an IIFE `self.requirejs` is undefined.

addon/index.ts

@@ -1,3 +1,9 @@
+declare global {
+  var requirejs: {
+    _eak_seen: Object;
+  };
+}
+
 import Engine from '@ember/engine';
 import require from 'require';
 
@@ -39,7 +45,7 @@ export default function loadInitializers(app: typeof Engine, prefix: string): vo
   var instanceInitializers = [];
   // this is 2 pass because generally the first pass is the problem
   // and is reduced, and resolveInitializer has potential to deopt
-  var moduleNames = Object.keys(self.requirejs._eak_seen);
+  var moduleNames = Object.keys(requirejs._eak_seen);
   for (var i = 0; i < moduleNames.length; i++) {
     var moduleName = moduleNames[i];
     if (moduleName.lastIndexOf(initializerPrefix, 0) === 0) {