Change how ListUnpackedPackagesSlice works.
Change how ListUnpackedPackagesSlice works.

Instead of just listing everything under BASE_DIR/*/*, wich leads to
false-positives on a real system (i.e. one with a populated
/usr/local/bin), attempt to load a metadata.json file, and if it does
not exist, then just skip it.

references #2

nesv/pm,nesv/pm