feat: improve extending github layers

[pi0]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR improves extending behavior for GitHub layers by preferring to clone to local node_modules/.c12/{id}. This way layers can share the same dependencies of the main project.

Other improvements:

• Cloned layer names are explicitly hashed using ohash. This resolves issues when cloning from same github remote but different directories when it was possible to conflict cache
• Supported gh: shorthand

📝 Checklist

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

[codecov]

Codecov Report

Merging #109 (34f95eb) into main (7940e9b) will increase coverage by 0.37%.
Report is 1 commits behind head on main.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   77.55%   77.93%   +0.37%     
==========================================
  Files           5        5              
  Lines         704      716      +12     
  Branches       63       64       +1     
==========================================
+ Hits          546      558      +12     
  Misses        158      158              

Files	Coverage Δ
src/loader.ts	90.18% <91.66%> (+0.38%)	⬆️