ISSUE: 判断重复import的逻辑太粗暴，性能可能有问题

            // path += L".wo";
            std::wstring srcfile, src_full_path;
            if (!wo::read_virtual_source(&srcfile, &src_full_path, path + L".wo", wo::wstr_to_str(*lex.source_file).c_str()))
            {
                // import a::b; cannot open a/b.wo, trying a/b/b.wo
                if (!wo::read_virtual_source(&srcfile, &src_full_path, path + L"/" + filename + L".wo", wo::wstr_to_str(*lex.source_file).c_str()))
                    return token{ lex.parser_error(lexer::errorlevel::error, WO_ERR_CANNOT_OPEN_FILE, path.c_str()) };
            }

            if (!lex.has_been_imported(wstring_pool::get_pstr(src_full_path))
                && !lex.has_been_imported(wo::crc_64(srcfile.c_str(), 0)))
            {