fix: improve package.json detection

[re-taro]

Summary

• Fix issue where root-level package.json was not being detected
• The condition item.path.endsWith("/package.json") doesn't match package.json at the root directory, so added item.path === "package.json" check

Changes

• get-changed-packages.ts: Add condition to properly detect root-level package.json

[vercel]

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