core: don't load dropin data multiple times for the same unit (#5139)

When an alias is loaded, we resolve this alias to its final unit first to load
the dropin data.

However if the final unit was already loaded, there's no point in reloading the
dropin data a second time.

This patch optimizes this case.

Also this allows the dropin loading code to assume that only units not yet
loaded are passed down. This assumption is not yet used but might be in the
future.

[zj: invert the condition in the if]