[Refactor] Use uniq helper in import-extractor

[gonzaloriestra]

WHY are these changes introduced?

Improving code consistency and readability by using existing utility helpers.

WHAT is this pull request doing?

Replaced manual new Set spreads with the uniq utility from @shopify/cli-kit/common/array in packages/cli-kit/src/public/node/import-extractor.ts.

How to test your changes?

CI

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

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PR created automatically by Jules for task 12023691149304443027 started by @gonzaloriestra

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