Add protective diversions for netplan-generator usrmerge (Closes: #1060661)

This MR is addressing the usrmerge transition. In bookworm we had the netplan.io binary package ship the file lib/systemd/system-generators/netplan. Now, the package was split and we have the netplan-generator binary shipping the file usr/lib/systemd/system-generators/netplan instead.

This can lead to trouble, according to Dep17.P1 and we're mitigating it using DEP17.M8