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/flash/flash.py

@@ -93,9 +93,7 @@ def flash_machine(
 
         from clan_cli.vars.generate import Generator
 
-        for generator in Generator.generators_from_flake(
-            machine.name, machine.flake, machine
-        ):
+        for generator in Generator.generators_from_flake(machine.name, machine.flake):
             for file in generator.files:
                 if file.needed_for == "partitioning":
                     msg = f"Partitioning time secrets are not supported with `clan flash write`: clan.core.vars.generators.{generator.name}.files.{file.name}"