vars: implement migration

Migrating generated files from the facts subsystem to the vars subsystem is now possible.

HowTo:
1. declare `clan.core.vars.generators.<generator>.migrateFact = my_service` where `my_service` refers to a service from `clan.core.facts.services`
2. run `clan vers generate your_machine` or `clan machines update your_machine`

Vars will only be migrated for a generator if:
1. The facts service specified via `migrateFact` does exist
2. None of the vars to generate exist yet
3. All public var names exist in the public facts store
4. All secret var names exist in the secret fact store

If the migration is deemed possible, the generator script will not be executed. Instead the files from the public or secret facts store are read and stored into the corresponding vars store

nixosModules/clanCore/vars/default.nix

@@ -42,6 +42,7 @@ in
           inherit (generator)
             dependencies
             finalScript
+            migrateFact
             prompts
             share
             ;

nixosModules/clanCore/vars/interface.nix

@@ -12,6 +12,7 @@ let
     either
     enum
     listOf
+    nullOr
     package
     path
     str
@@ -49,6 +50,16 @@ in
               type = listOf str;
               default = [ ];
             };
+            migrateFact = {
+              description = ''
+                The fact service name to import the files from.
+
+                Use this to migrate legacy facts to the new vars system.
+              '';
+              type = nullOr str;
+              example = "my_service";
+              default = null;
+            };
             files = {
               description = ''
                 A set of files to generate.