resolved.service: set DefaultDependencies=no

On systems that only use resolved for name resolution, there are usecases that
require resolved to be started before sysinit target, such that network name
resolution is available before network-online/sysinit targets. For example,
cloud-init for some datasources hooks into the boot process ahead of sysinit
target and may need network name resolution at that point already.

systemd-resolved already starts pretty early in the process, thus starting it
slightly earlier should not have negative side effects.

However, this depends on resolved ability to connect to system DBus once that
is up.