android_system_image used GatherPackagingSpecs and then filter only
system modules. But some modules were omitted in this logic because
there are modules which has the same relative path, so the later one is
ignored even though its partition info is what we're looking for. So add
filter logic in GatherPackagingSpecs to avoid this problem
Bug: 323793487
Test: build android_system_image, and then check if it contains every
module we want
Change-Id: Iec8ae920736d3d1920eecad71ba0f8f2fe848e6c
54bf875c97