units: systemd-resolved should start before network-online.target and nss-lookup.target (#5691)

systemd-resolved provides
1. local API via NSS and D-Bus
2. kind of a local "DNS proxy" through its stub listener
The 1st item should be started before nss-lookup.target.
The 2nd item should be started before network-online.target,
because if the networking works in general, then DNS (and DNS proxy) should too.

Fixes #5650