feat(externals): support aliasing traced packages

[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

Normally when using the resolutions field to use an aliased package (say original name is a and aliased from npm:b), the package manager downloads b with and copies all contents into node_modules/a (while node_modules/a/package.json is package with name: b.). We use "directory" name as authentic package name to copy into .output/server/node_modules

pnpm also does this but there is a catch! the node_modules/a is a symbolic link to node_modules/.pnpm/b. When we are running external tracer, we follow the links and assume the real name is b which is wrong. To add to the complexity, rollup, mlly and nft also resolve real paths in various places so it is almost impossible to trace aliaces without manually specifying. (We might auto detect from root level package.json using resolutions and aliases)

This PR enables new externals.traceAliases option in order to speficy trace aliased packages:

export default defineNitroConfig({
  externals: {
    traceAlias: {
      "h3-nightly": "h3"
    },
  },
})

This tells nitro when tracing .output/server/h3-nightly, try to link it to .output/server/node_modules/h3.

(h3-nightly alias is added by default as a known dependency)

Isolated Preset Testing

This PR enables a new isolated way of testing presets.

Previously, we were running fixture build output within /<repo>/test/presets/.tmp/node. Even if an external like h3 is missing, tests were wrongly passed since Node.js also searches in /<repo>/node_modules. The tests are now more secure by using /tmp/nitro-test/<preset>/output.

📝 Checklist

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

[codecov]

Codecov Report

Merging #1553 (3d6737e) into main (de6d2d0) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1553      +/-   ##
==========================================
- Coverage   76.47%   76.43%   -0.04%     
==========================================
  Files          73       73              
  Lines        7574     7580       +6     
  Branches      750      751       +1     
==========================================
+ Hits         5792     5794       +2     
- Misses       1781     1784       +3     
- Partials        1        2       +1     

Files Changed	Coverage Δ
src/rollup/config.ts	89.77% <100.00%> (-0.76%)	⬇️
src/rollup/plugins/externals.ts	93.29% <100.00%> (+0.02%)	⬆️