This enhances #61423, which removed the gating of desktop-managers from
being linked to the xserver's state.
This, though, brought in xterm into all systems, even those without X
servers.
This change sets the *default* of the xterm desktop-manager to the state
of the xserver, keeping it enabled by default as a sane fallback.
The xterm desktop-manager can still be enabled or disabled as needed,
without it being affected by xserver's state.
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.
