error while loading shared libraries: libsystemd.so.0: cannot open shared object file: No such file or directory
In many attempts to install pkgs (not from AUR) I get this message
In particular conky will not work no matter what I do, but I have gotten the same message before.

Is there a way to work around this? Can't there be like a dummy substitute library, or is there a way to edit pkgs to point to a different library?
you need to parse the PKGBUILD and find the option which enable systemd like e.g this https://forum.obarun.org/viewtopic.php?id=137 post number 3.
A workaround can be 
# pacman -S --assume-installed systemd --assume-installed libsystemd
obviously this will install the package but if the package need system:D for the running time, you risk to have strange behaviour.

you can install it removing dependencies checking repeating twice the option -d 
# pacman -Sdd foo
The best (and only manner if you want a solid system) to have a proper package,it is to compile it without system:D dependencies on it. As far as know the only one which cannot be compiled without system:D is libgdm. The rest of the package can be compiled without system:D.

Some package have ghosted system:D dependency as mpv which need samba which need system:D...

The other solution are not clean as installing dummy package. This is the best manner to have obscur bug on your system. Fake library are not real library, so package can install but the running time can give you strange behaviour and can be horrible to debug cause of the false output debug message from the soft. The message that you post in your message is very clear and easy to debug... not the same with dummy package.

for conky, on the pkgbuild you have: 
-D BUILD_JOURNAL=ON
turn it off and rebuild, really simple.
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