esm: resolve main from package.json when importing from directory

[lingsamuel]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Fixes #32103
Ref: My issue comment

[guybedford]

The Node.js modules documentation tries to make it clear that explicit relative paths are always required. See https://nodejs.org/dist/latest-v13.x/docs/api/esm.html#esm_differences_between_es_modules_and_commonjs.

If you can suggest improvements to these docs to avoid any misunderstandings that would be very welcome.

Alternatively you can use the --es-module-specifier-resolution=node flag here.

[MylesBorins]

@guybedford I think this is meant to specifically fix a bug in the --experimental-specifier-resolution flag

[guybedford]

@lingsamuel my sincere apologies for misreading this PR - the change is definitely a bug fix for the flagged support.

My main review point further is that this check should be combined into the finalizeResolution code path as a unified LOAD_AS_DIRECTORY routine, as per https://nodejs.org/dist/latest-v12.x/docs/api/modules.html#modules_all_together.

[guybedford]

I think this should probably combined into the finalizeResolution function above rather to maintain full equivalence with the LOAD_AS_FILE | LOAD_AS_DIRECTORY checks per the module resolution spec.

[lingsamuel]

The first version I wrote puts this part in finalizeResolution function indeed. But I think finalizeResolution is LOAD_AS_FILE routine, it is called by many other resolve functions as final return call and shouldn't add any resolve functionality. So I move this code pieces after the relative path check to fit this rule:

3. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)`

[guybedford]

It is ok for finalizeResolution to include the LOAD_AS_FILE | LOAD_AS_DIRECTORY path - the reason for this is that all resolutions eventually end in this check in CommonJS.

[lingsamuel]

Sorry for my late reply.
I don't think it's a good idea...
finalizeResolution is pretty simple now, and keep it simple is better to my mind. Merge package resolve logic to it makes it a bit dirty, the call path does not straight anymore.
But if more people think merge logic is better, I will change it...

[guybedford]

@lingsamuel let me restate my feedback then. This PR only fixes one case, when there are two other test cases not being covered by this PR that you should make pass as well as part of this fix:

1. import 'pkg/x' should load /path/to/node_modules/pkg/x/pjson-main.js with package.json main checks
2. import 'pkg/x' where pkg/package.json contains { "exports": { "./x": "./z" } should load /path/to/node_modules/pkg/z/pjson-main.js

[aduh95]

Superseded by #38979.