Release 1.2.1: Fix duplicate import handling

Summary

Fixes duplicate import handling for horizon-supported-systems.flakeModule.

Changes

• Add flake-parts.flakeModules.flakeModules import to enable proper module deduplication
• Move flake.flakeModule export into flakeModule.nix (like gitlab-ci does)
• Remove separate nix/exports.nix file
• Add test/import-deduplicate/ test to verify duplicate imports work
• Reorganize tests into test/basic/ and test/import-deduplicate/
• Update CI to test both test directories

Details

The fix adds local.inputs.flake-parts.flakeModules.flakeModules to the imports before importAndPublish, which enables proper module deduplication. This allows horizon-supported-systems.flakeModule to be imported multiple times without "already declared" errors.

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