refactor: use unimport built-in type gen for dirs

[antfu]

🔗 Linked issue

close #877

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov]

Codecov Report

Merging #994 (713410e) into main (e08a8e9) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #994      +/-   ##
==========================================
- Coverage   67.79%   67.73%   -0.07%     
==========================================
  Files          60       60              
  Lines        6133     6127       -6     
  Branches      692      690       -2     
==========================================
- Hits         4158     4150       -8     
- Misses       1966     1967       +1     
- Partials        9       10       +1     

Impacted Files	Coverage Δ
src/build.ts	44.62% <0.00%> (-1.45%)	⬇️

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