Remove deprecated property usage

The deprecation warning appears to cause build failures in some cases.

lib/AmdMainTemplatePlugin.js

@@ -17,7 +17,7 @@ class AmdMainTemplatePlugin {
 		const mainTemplate = compilation.mainTemplate;
 
 		compilation.templatesPlugin("render-with-entry", (source, chunk, hash) => {
-			const externals = chunk.modules.filter((m) => m.external);
+			const externals = chunk.getModules().filter((m) => m.external);
 			const externalsDepsArray = JSON.stringify(externals.map((m) =>
 				typeof m.request === "object" ? m.request.amd : m.request
 			));