api/machines: allow importing extra modules

- add top-level option `clanImports` to clanCore
- clanImports can be set and checked as any other option
- buildClan resolves the clanImports from the settings.json before calling evalModules to prevent infinite recursions
- new endpoint PUT machines/{name}/schema to allow getting the schema for a specific list of imports
- to retrieve the currently imported modules, cimply do a GET or PU on machines/{name}/config which will return `clanImports` as part of the config

Still missing: get list of available modules

lib/flake-module.nix

@@ -8,7 +8,7 @@
   ];
   flake.lib = import ./default.nix {
     inherit lib;
-    inherit self;
     inherit (inputs) nixpkgs;
+    clan-core = self;
   };
 }