Redesign validators with mkValidatorModule

• Replace validate module with individual validator modules
• Add mkValidatorModule helper function for creating validators
• Convert flake-lock to standalone validator module
• Fixes issue where validator default values weren't being applied

Each validator is now self-contained and independently importable. Third parties can create validators using mkValidatorModule.

New usage:

imports = [ inputs.gitlab-ci.modules.gitlab-ci.flake-lock ];
flake-lock.enable = true;

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