systemd: Only merge /lib/systemd/system into /usr for now

Simon McVittierequested to merge
smcv/debhelper:systemd-generators into main

systemd and init-system-helpers read both /lib/systemd/system and /usr/lib/systemd/system for units, so it's OK to normalize everything to be installed into /usr/lib/systemd/system.

However, the same is not true for other subdirectories of /lib/systemd, such as /lib/systemd/system-generators. Until the systemd maintainers confirm which files they would like to be migrated, and whether a ${misc:Depends} on a newer systemd is required for the transition, it seems like the safest thing to do is to restrict this merging to just the units from /lib/systemd/system.

Closes: #992554

Seems to work when testing against ostree.

Edited by Simon McVittie

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