How should I do when two package be dependent on each other?

In reply to designer007:

//top.sv

import base::;
import high::
;

`include “testcase0.sv” //if testcase0 not include in base pck, I think this way works

module top()

endmodue

before this way, I try to add testcase0 in the filelist.f
add always get errors.
//filelist.f
./base.sv
./high.sv
./testcase0.sv
./top.sv