Add the dbus service file in the package folder to XDG_DATA_HOME, as
well as adding an assertion to ensure both it and `gnome-keyring` aren't
enabled at the same time.
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
