core: do not leak resources when handling stale alias state on reload

core: do not leak resources when handling stale alias state on reload

The fix for the corrupted state when units become aliased on reload
leaks the now-aliased unit's resources, which become untracked and
essentially lost.

While fixing the state corruption is of course necessary, leaking
processes/etc. is not ideal for a system and service manager, so
instead attempt to keep track of them by creating stub units
on-the-fly as replacements.
This way resources are not leaked, there are clear indications of
where they moved, and all state can be tracked as expected.

Ignore timers/sockets/paths as those are internal resources/triggers.

Follow-up for a77c7a8224447890a304bd857f412c8103f217f1