core: introduce Unit.dependency_generation counter and restart loop when dependency...

core: introduce Unit.dependency_generation counter and restart loop when dependency is updated in the loop

When starting unit A, a dependent unit B may be loaded if it is not
loaded yet, and the dependencies in unit A may be updated.
As Hashmap does not allow a new entry to be added in a loop, we need to
restart loop in such case.

Fixes a bug introduced by cda667722c2218cf1a0185284d2a87f8a25f1b2d.
Fixes #36031.