core: don't drop transient drop-ins when loading the rest

core: don't drop transient drop-ins when loading the rest

Previously, when creating a transient unit, we'd first add the transient
drop-ins to the unit, and then normally load any other drop-ins later on top of
this, replacing the already loaded drop-ins. Let's not do this, after all the
transient drop-ins area already in effect, let's just add what we find on disk,
but not replace it.