With a service with DynamicUser= with static user or group, e.g.,
```
$ systemd-run -p DynamicUser=yes -p Group=disk sleep infinity
```
previously the lookup by name and ID through DBus/Varlink are inconsistent:
```
$ busctl call org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager LookupDynamicUserByUID "u" 6
Call failed: Dynamic user ID 6 does not exist.
$ busctl call org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager LookupDynamicUserByName "s" disk
u 6
$ userdbctl group 6
Group name: disk
Disposition: system
GID: 6
Passwords: 1
Service: io.systemd.NameServiceSwitch
$ userdbctl group disk
Group name: disk
Disposition: dynamic
GID: 6
Description: Dynamic Group
Service: io.systemd.DynamicUser
```
With this change, the results of these methods are consistent.
Fixes#40228.
Yu Watanabe
cb39d66ec8