feat(flake)!: convert to fs-defined modules

Place all modules in either:
    - `modules/${manager}/${module}.nix`
    - `modules/${manager}/${module}/default.nix`

- `${manager}` - One of `nixos`, `nix-on-droid`, `nix-darwin` or `home-manager`
- `${module}` - Module name (autoimported by configurations, used for exporting)

modules/home-manager/README.md

@@ -0,0 +1,5 @@
+Add your reusable home-manager modules to this directory (<https://nixos.wiki/wiki/Module>)
+
+These are considered modules:
+- files, ending in `.nix` (excluding `default.nix`)
+- directories, containing `default.nix`