module: fix extensionless CJS files in type:module packages

[mcollina]

Summary

• PR module: fix extensionless entry with explicit type=commonjs #61600 made the CJS loader respect package.json type field for extensionless files, but also enforced type: "module" which breaks CJS extensionless files inside ESM packages (e.g. yargs v17)
• Only enforce type: "commonjs" for extensionless files; for type: "module", leave format undefined so V8's syntax detection determines the correct format
• Adds regression test that require()s a CJS extensionless file from a type: "module" package and verifies exports are correct

Test plan

• Existing test test/es-module/test-extensionless-esm-type-commonjs.js still passes (ESM syntax in type: "commonjs" still throws SyntaxError)
• New test case verifies CJS extensionless file in type: "module" package returns correct exports via require()
• Manual verification: require() of fixture returns { hello: 'world' } (was returning empty ESM namespace before fix)

Fixes: #61971
Refs: yargs/yargs#2509
Refs: #61600

[nodejs-github-bot]

Review requested:

• @nodejs/loaders

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.65%. Comparing base (e78bf55) to head (519235b).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #62083   +/-   ##
=======================================
  Coverage   89.64%   89.65%           
=======================================
  Files         676      676           
  Lines      206249   206326   +77     
  Branches    39518    39527    +9     
=======================================
+ Hits       184892   184980   +88     
+ Misses      13479    13471    -8     
+ Partials     7878     7875    -3     

Files with missing lines	Coverage Δ
lib/internal/modules/cjs/loader.js	98.14% <100.00%> (-0.22%)	⬇️

... and 32 files with indirect coverage changes

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[joyeecheung]

Code LGTM with a correction on comment

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71545/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71548/

[GeoffreyBooth]

• PR module: fix extensionless entry with explicit type=commonjs #61600 made the CJS loader respect package.json type field for extensionless files, but also enforced type: "module" which breaks CJS extensionless files inside ESM packages (e.g. yargs v17)
• Only enforce type: "commonjs" for extensionless files; for type: "module", leave format undefined so V8's syntax detection determines the correct format

I think this proposed behavior is incorrect; according to our spec, extensionless files within a package scope with an explicit type field follow the format of the type field—that is, they behave however .js files would behave. We never do syntax detection when there's an explicit type field.

I read yargs/yargs#2509 and it's unfortunate that they seemed to rely on what was buggy behavior. But this PR seems to make several breaking changes, going against our own documented behaviors for how the type field works and how syntax detection works. I'm not sure what to do instead; perhaps just revert #61600 and re-land it as semver-major?

Marking as request changes so that we discuss this before landing. If others disagree with my analysis here I'm happy to be persuaded I'm wrong; I understand there's probably an eagerness to unbreak a popular library but let's not be so hasty that we cause other bugs.