refactor: reduce coupling to Machine class in vars module

- Change Generator class to store machine name as string instead of Machine reference
- Update Generator.generators_from_flake() to only require machine name and flake
- Refactor check_vars() to accept machine name and flake instead of Machine object
- Create Machine instances only when needed for specific operations

This continues the effort to reduce dependencies on the Machine class,
making the codebase more modular and easier to refactor.

pkgs/clan-cli/clan_cli/vars/fix.py

@@ -11,7 +11,7 @@ log = logging.getLogger(__name__)
 def fix_vars(machine: Machine, generator_name: None | str = None) -> None:
     from clan_cli.vars.generate import Generator
 
-    generators = Generator.generators_from_flake(machine.name, machine.flake, machine)
+    generators = Generator.generators_from_flake(machine.name, machine.flake)
     if generator_name:
         for generator in generators:
             if generator_name == generator.name: