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

pkgs/clan-cli/clan_cli/machines/machines.py

@@ -10,6 +10,8 @@ from typing import Any, Literal
 from clan_cli.clan_uri import FlakeId
 from clan_cli.cmd import run_no_stdout
 from clan_cli.errors import ClanError
+from clan_cli.facts import public_modules as facts_public_modules
+from clan_cli.facts import secret_modules as facts_secret_modules
 from clan_cli.nix import nix_build, nix_config, nix_eval, nix_metadata
 from clan_cli.ssh import Host, parse_deployment_address
 from clan_cli.vars.public_modules import FactStoreBase
@@ -85,6 +87,16 @@ class Machine:
     ]:
         return self.deployment["facts"]["publicModule"]
 
+    @cached_property
+    def secret_facts_store(self) -> facts_secret_modules.SecretStoreBase:
+        module = importlib.import_module(self.secret_facts_module)
+        return module.SecretStore(machine=self)
+
+    @cached_property
+    def public_facts_store(self) -> facts_public_modules.FactStoreBase:
+        module = importlib.import_module(self.public_facts_module)
+        return module.FactStore(machine=self)
+
     # WIP: Vars module is not ready yet.
     @property
     def secret_vars_module(self) -> str: