BUG: if providing `main` field instead of exports `findFirstExistingMainFieldMappedFile` fails silently

[shairez]

if pointing to a path of a library which has a package.json that has a main property and not exports property, the function findFirstExistingMainFieldMappedFile ignores the package.json and doesn't actually continues to figure out if there's a main field...

it also doesn't work when using:

"exports": "./index.js"

only with:

"exports": {
  ".": "./index.js"
}

I can fix this with a PR but before I do I wanted to verify that it has a chance to get merged

Thanks for this library! 🙏