Add feature to know if schema is imported in avdl

@namespace("purchase")
protocol InvoiceProtocol{
   import idl "../customer/data.avdl";  
   import idl "../product/data.avdl";    
   record Invoice{
      timestamp_ms purchaseTime;
      customer.Customer customer;
      array<product.Item> items = [];
   }
}

for(Schema s : protocol.getTypes()){
   System.out.println(s.getName() + " " + s.isImported());
}

Customer true
Item true
Invoice false