Merge the `customize-*.nix` modules with their base modules.

Currently, the modules in morph/lib are split in two, with a static module, and a function returning a module (i.e. the customize-*.nix files) which has the dynamic parts.

However, the idiom that NixOS uses for this kind of configuration is to define settings in a module and then have the configuration of the module be derived from those settings and settings from other modules.

Doing this will let us combine the static and dynamic modules, which means we need to look at less files to see the the shape of the final configuration.