refactor top level declarations

- add topLevelDeclarations to code generation results
- set topLevelDeclarations=undefined if any child topLevelDeclarations=undefined

lib/library/AssignLibraryPlugin.js

@@ -243,7 +243,7 @@ class AssignLibraryPlugin extends AbstractLibraryPlugin {
 		if (topLevelDeclarations.has(base))
 			return topLevelDeclarationContradictionError(base);
 		const { data } = codeGenerationResults.get(module, chunk.runtime);
-		topLevelDeclarations = data.get("topLevelDeclarations");
+		topLevelDeclarations = data && data.get("topLevelDeclarations");
 		if (topLevelDeclarations && topLevelDeclarations.has(base)) {
 			return topLevelDeclarationContradictionError(base);
 		}