sys_plugins table will give you the plugins details , example: table details or script details has been used in perticular plugins , but if you will go to the v_plugin then you will find the plugins details as well as you will able to activate it . 

i believe sys_plugins stores all the plugins available in service-now, some of them are accessible from service-now and some of them are accessible through hi request. 

now coming to the question you had asked to get the list of active plugins , now in servicenow it has new page which show the plugins or you can categories them base on modules or active or inactive. 

screenshot below.