This will reduce the system closure size by about 200MB under NixOS by
sharing the glibcLocales package.
When home-manager is installed on Linux without the NixOS module, all
glibc locales are installed, as before.
Resolves: #2333
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
