Do not use global imports of packages in the compilation unit scope unless necessary

Created by: Silabs-ArjanB

The use of global imports of packages (as for example done here https://github.com/openhwgroup/cva6/blob/c57b275df595601cae85eed6b3b4d47639a6f510/src/ariane.sv#L25) should be avoided. For further justification please see the corresponding issue in CV32E40P https://github.com/openhwgroup/cv32e40p/issues/384. In CV32E40P all such imports were fixed in https://github.com/openhwgroup/cv32e40p/pull/389 and ideally the same is done in CVA6.