`import/extensions` flagging when default package export is a json file

I have a package with a default export of `index.json` instead of `index.js`. When I try to import this package:
```
import foo from 'json-package';
```
`import/extensions` complains about the lack of an extensions. This doesn't seem like desired behavior.

fyi: @ljharb

Start by reproducing the `import/extensions` report with a package whose default export is `index.json` and the shown import. Read the rule's handling of package entry points and compare it with the expected behavior for JavaScript exports. Done means the valid JSON package import is handled correctly and existing extension checks remain covered.

Written by the indexing model from the issue text.