core: delay adding target dependencies until all units are loaded and aliases resolved (#8381)

Currently we add target dependencies while we are loading units. This
can create ordering loops even if configuration doesn't contain any
loop. Take for example following configuration,

$ systemctl get-default
multi-user.target

$ cat /etc/systemd/system/test.service
[Unit]
After=default.target

[Service]
ExecStart=/bin/true

[Install]
WantedBy=multi-user.target

If we encounter such unit file early during manager start-up (e.g. load
queue is dispatched while enumerating devices due to SYSTEMD_WANTS in
udev rules) we would add stub unit default.target and we order it Before
test.service. At the same time we add implicit Before to
multi-user.target. Later we merge two units and we create ordering cycle
in the process.

To fix the issue we will now never add any target dependencies until we
loaded all the unit files and resolved all the aliases.

src/core/manager.h

@@ -144,6 +144,9 @@ struct Manager {
         /* Units whose cgroup ran empty */
         LIST_HEAD(Unit, cgroup_empty_queue);
 
+        /* Target units whose default target dependencies haven't been set yet */
+        LIST_HEAD(Unit, target_deps_queue);
+
         sd_event *event;
 
         /* This maps PIDs we care about to units that are interested in. We allow multiple units to he interested in