add note about webpack 5 refactoring

lib/dependencies/DependencyReference.js

@@ -7,6 +7,9 @@
 /** @typedef {import("../Module")} Module */
 
 class DependencyReference {
+	// TODO webpack 5: module must be dynamic, you must pass a function returning a module
+	// This is needed to remove the hack in ConcatenatedModule
+	// The problem is that the `module` in Dependency could be replaced i. e. because of Scope Hoisting
 	/**
 	 *
 	 * @param {Module} module the referenced module
@@ -15,6 +18,7 @@ class DependencyReference {
 	 * @param {number} order the order information or NaN if don't care
 	 */
 	constructor(module, importedNames, weak = false, order = NaN) {
+		// TODO webpack 5: make it a getter
 		this.module = module;
 		// true: full object
 		// false: only sideeffects/no export

lib/optimize/ConcatenatedModule.js

@@ -429,6 +429,7 @@ class ConcatenatedModule extends Module {
 				})
 				.filter(ref => ref);
 			DependencyReference.sort(references);
+			// TODO webpack 5: remove this hack, see also DependencyReference
 			return references.map(ref => {
 				const dep = map.get(ref);
 				return () => dep.getReference().module;