Before, loading a module would be guarded by an optional platform
condition. This made it possible to avoid loading and evaluating a
module if it did not support the host platform.
Unfortunately, this made it impossible to share a single configuration
between GNU/Linux and Darwin hosts, which some wish to do.
This removes the conditional load and instead inserts host platform
assertions in the modules that are platform specific.
Fixes#1906
Nixpkgs no longer packages compton, and instead packages picom, a
(mostly) compatible fork of compton, providing an alias from compton
to picom. Because some configuration options have been changed, and
all references to "compton" have been made deprecated and replaced
with "picom", 'services.compton' has been deprecated in favor of the
new 'services.picom'.
Resolves#878
PR #1101
