[pm][delta] Separate source and target filters

Allow the filtering step of a package snapshot to use different filters
for the source and target snapshots, to, for example, allow showing the
side effects of adding or removing a single package to an OS image,
without having to actually produce 2 builds.

Test: manual
Change-Id: Iceb7d6fcdd7caca0dead6aa176527e0bb69cf289
1 file changed