It seems that with the latest update to `udisks2`, the ExecStart path
for the daemon changed from `/lib/udisks2` to `/libexec/udisks2`. This
commit reflects that change for our purposes.
The ability for unprivileged users to mount external media is useful
regardless of the desktop environment. Also, since udisks2 is
activated on-demand, it doesn't add any overhead if you're not using it.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
