Inventory: allow arbitrary imports from nix

lib/inventory/build-inventory/default.nix

@@ -145,7 +145,7 @@ let
               builtins.map (role: serviceConfig.roles.${role}.config or { }) inverseRoles.${machineName} or [ ]
             );
 
-            customImports = map (s: "${directory}/${s}") (
+            customImports = map (s: if builtins.typeOf s == "string" then "${directory}/${s}" else s) (
               globalImports ++ machineImports ++ roleServiceImports
             );
           in

lib/inventory/build-inventory/interface.nix

@@ -61,7 +61,12 @@ let
 
     '';
     default = [ ];
-    type = types.listOf types.str;
+    type = types.listOf (
+      types.oneOf [
+        types.str
+        types.anything
+      ]
+    );
   };
 in
 {