install: allow adding plain templates to .wants/ or .requires/

install: allow adding plain templates to .wants/ or .requires/

Fixes #19437.

As reported in the bug:

> # drkonqi-coredump-processor@.service
>  ...
> [Install]
> WantedBy=systemd-coredump@.service
>
> The plan here is to have a systemd-coredump@ instance start the same %i for
> drkonqi-coredump-processor@. Works perfectly when creating the symlink manually
> ln -sv /usr/lib/systemd/system/drkonqi-coredump-processor@.service
> /etc/systemd/system/systemd-coredump@.service.wants/.

When DefaultInstance is set, we replace template references with
template@default-inst. But in this case we want to create a symlink for the
template name, so that systemd will fill in the instance from the
wanting/requiring unit. This is only possible for those units that actually
have an instance set, so we create the symlink only from .requires/ or .wants
of an instantiated unit (then this specific instance will be used), or a
template (than some instance will be inherited later).

Specifically:
...
[Install]
WantedBy=other@.service, fixed.service
DefaultInstance=inst

→ enable foo@.service creates other@.service.wants/foo@inst.service, and
other@a.service will want foo@inst.service, and other@b.service will want foo@inst.service,
and fixed.service will want foo@inst.service.

Without DefaultInstance,
→ enable foo@.service creates other@.service.wants/foo@.service, and
other@a.service would want foo@a.service, and other@b.service would want foo@b.service,
but enablement fails because no dependency can be created for fixed.service:

  Failed to enable unit, unit fixed.service is a non-template unit.