Ability to symlink files to `/lib` and `/usr/lib`

[giggio]

Is your feature request related to a problem? Please describe.

I'd like to be able to simply add a script to /lib/ or any subdirectory.

Describe the solution you'd like

Very similar to what is currently available to /etc:

      lib.systemd.system-sleep = {
        foo = {
          text = ''
            This is just a test!
          '';
          target = "foo_test";
        };

Describe alternatives you've considered

N/A.

Additional context

I want to add scripts to /lib/systemd/system-sleep/ to setup sleep hooks. See this issue on systemd issue tracker to understand why it is not on /etc.

I'm adding /usr/lib to this issue because it serves a similar purpose on a Linux system. On Ubuntu, they are symlinked.

Related: #297.