fix: suffix export of runtime utils with /index

[DamianGlowala]

🔗 Linked issue

resolves #1460

❓ 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)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Likely related to nuxt/module-builder#308.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.21%. Comparing base (e1a8cea) to head (e3befbf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1467   +/-   ##
=======================================
  Coverage   63.21%   63.21%           
=======================================
  Files          79       79           
  Lines        3616     3616           
  Branches      406      407    +1     
=======================================
  Hits         2286     2286           
  Misses       1301     1301           
  Partials       29       29           

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