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
Add new options Darwin options:
- `targets.darwin.defaults`
This adds options for configuring macOS through the `defaults(1)` system.
This option can be used to manipulate a vast majority of user settings for macOS
and its applications.
This is implemented using freeform modules and includes additional descriptions
and type information for some useful options.
- `targets.darwin.keybindings`
This adds options for configuring the default keybindings for macOS text fields.
- `targets.darwin.search`
This adds options for configuring the default search engine for macOS.
