modules: refactor module bootstrapping

Let's simplify things by defining all modules in `./plugins`, `./modules`
and `./wrappers/modules`.

Instead of currying `pkgs` into a bootstrapping module, we can require
`defaultPkgs` be provided as a special arg.

This refactor allows us to completely remove `flake-modules/modules.nix`!

flake-modules/packages.nix

@@ -3,13 +3,12 @@
     {
       pkgsUnfree,
       config,
-      rawModules,
       helpers,
       ...
     }:
     {
       packages = import ../docs {
-        inherit rawModules helpers;
+        inherit helpers;
         # Building the docs evaluates each plugin's default package, some of which are unfree
         pkgs = pkgsUnfree;
       };