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
