frontend/base: import hierarchical cells attributes

frontend/frontend_base.h

@@ -535,6 +535,9 @@ template <typename FrontendType> struct GenericFrontend
         // Do the submodule import
         auto type = impl.get_cell_type(cd);
         import_module(submod, name, type, mod_refs.at(type));
+        // Add current cell attributes to the imported module
+        impl.foreach_attr( cd, [&](const std::string &name, const Property &value)
+              { ctx->hierarchy[submod.path].attrs[ctx->id(name)] = value; } );
     }
 
     // Import the cells section of a module