systemd: expand functionality

[aanderse]

this PR is still an early WIP but i thought it best to publish my work as soon as possible in case:

• anyone else was interested in this functionality and wanted to collaborate instead of duplicate work
• numtide absolutely doesn't want this functionality so they can tell me sooner rather than later

---

this PR adds a few more options/implements functionality from nixos:

• systemd.user.* options
• an implementation for systemd.packages so you can utilize upstream systemd units that live in nixpkgs packages

the latter is particularly useful to a large audience

testing and further details to follow, though as mentioned above... if this functionality is absolutely not wanted by numtide i would love to hear sooner rather than later ❤️

[aanderse]

update: i ended up testing both of these features and they work great. i will try to find some time to write a test for this and then mark as ready for review.

[aanderse]

any hint on how to interactively run the vm tests locally? when i run vm tests locally they always fail on this line, even against the numtide/main branch... 🤔

cc @r-vdp

[r-vdp]

@aanderse you can do that as follows:

nix build -vL '.#checks.x86_64-linux.vm-test-example-ubuntu_23_04_cloudimg.driverInteractive'
./result/bin/run-vm

and then you'll get the usual TUI for interactive NixOS tests.

From there, you can start the nodes and run the test script with

start_all()
test_script()

and you have things like node.shell_interact() to get a shell on a node.

I just tested this on the main branch and it works for me. Let me know if you're still stuck!