core: handle lookup paths being symlinks

core: handle lookup paths being symlinks

With a recent change paths leaving the statically known lookup paths would be
treated differently then those that remained within those. That was done
(AFAIK) to consistently handle alias names. Unfortunately that means that on
some distributions, especially those where /etc/ consists mostly of symlinks,
would trigger that new detection for every single unit in /etc/systemd/system.
The reason for that is that the units directory itself is already a symlink.

Rebased-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>