installer: filter derived udebs from parent list

the existing logic just bundled the entire parent and derived udeb lists
together, ignoring the fact that there might thus be two instances of some
packages, and relying upon getting derived ones first and checking file
existence to avoid handling the overridden parent instances.

here we now actually filter the list of parent udebs to exclude packages
that are to be obtained from the derivative.

this enables avoiding the file existence checking

Partial fix for #952914

Gbp-Dch: Short

0 files changed, 0 insertions, 0 deletions