Flag ids that are modules

src/ExternalModule.js

@@ -8,6 +8,7 @@ export default class ExternalModule {
 		// Implement `Identifier` interface.
 		this.originalName = this.name = makeLegalIdentifier( id );
 		this.module = this;
+		this.isModule = true;
 
 		// Define the external module's name in the bundle scope.
 		bundle.scope.define( id, this );

src/Module.js

@@ -32,6 +32,7 @@ export default class Module {
 		this.bundle = bundle;
 		this.id = id;
 		this.module = this;
+		this.isModule = true;
 
 		// Implement Identifier interface.
 		this.name = makeLegalIdentifier( basename( id ).slice( 0, -extname( id ).length ) );