shared/install: resolve specifiers in Also=

shared/install: resolve specifiers in Also=

Test case:
[Install]
WantedBy= default.target
Also=getty@%p.service

$ ./systemctl --root=/ enable testing@instance.service
Created symlink /etc/systemd/system/default.target.wants/testing@instance.service → /etc/systemd/system/testing@.service.
Created symlink /etc/systemd/system/getty.target.wants/getty@testing.service → /usr/lib/systemd/system/getty@.service.
$ ./systemctl --root=/ disable testing@instance.service
Removed /etc/systemd/system/getty.target.wants/getty@testing.service.
Removed /etc/systemd/system/default.target.wants/testing@instance.service.

Fixes part of #4210.

Resolving specifiers in DefaultInstance seems to work too:
[Install]
WantedBy= default.target
DefaultInstance=%u

$ systemctl --root=/ enable testing3@instance.service
Created symlink /etc/systemd/system/default.target.wants/testing3@instance.service → /etc/systemd/system/testing3@.service.
$ systemctl --root=/ enable testing3@.service
Created symlink /etc/systemd/system/default.target.wants/testing3@zbyszek.service → /etc/systemd/system/testing3@.service.