core: refresh unit cache when building a transaction if UNIT_NOT_FOUND

core: refresh unit cache when building a transaction if UNIT_NOT_FOUND

When a command asks to load a unit directly and it is in state
UNIT_NOT_FOUND, and the cache is outdated, we refresh it and
attempto to load again.
Use the same logic when building up a transaction and a dependency in
UNIT_NOT_FOUND state is encountered.
Update the unit test to exercise this code path.