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.
E.g.
The unique option `fileSystems./.device' is defined multiple times, in `/etc/nixos/configuration.nix' and `/etc/nixos/foo.nix'.
This requires passing file/value tuples to the merge functions.
For instance, if time.timeZone is defined multiple times, you now get
the error message:
error: user-thrown exception: The unique option `time.timeZone' is defined multiple times, in `/etc/nixos/configurations/misc/eelco/x11vnc.nix' and `/etc/nixos/configuration.nix'.
while previously you got:
error: user-thrown exception: Multiple definitions of string. Only one is allowed for this option.
and only an inspection of the stack trace gave a clue as to what
option caused the problem.
