Ensure that loading packages can be extended

Collections.emptySet() creates an immutable collection which will prevent
us to add more packages to the loading packages set.

Instead of using the emptySet, create a new set everytime, even if it's
empty.

Test: PackageDexUseTest
Bug: 148774920
Change-Id: If274d0380d494d31768da24c10153810ef8bf513