In reply to dave_59:
Dave you are right. But putting the class a definition into package pa like this
package pa;
class a extends uvm_sequence_item;
.....
endclass
endpackage
package pb;
import pa::*;
export pa::a;
<more entries>
endpackage
class test extends base_test;
import pb::*;
a aobj;
.....
endclass