fix(ssdk): import handler from src folder (#438)

Author: AllanZhengYP

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/HttpProtocolTestGenerator.java

@@ -407,7 +407,7 @@ private void setupStubService(Symbol operationSymbol,
         });
 
         String getHandlerName = "get" + handlerSymbol.getName();
-        writer.addImport(getHandlerName, null, "./server/");
+        writer.addImport(getHandlerName, null, "./" + CodegenUtils.SOURCE_FOLDER + "/server/");
 
         if (!usesDefaultValidation) {
             writer.addImport("ValidationFailure", "__ValidationFailure", "@aws-smithy/server-common");