allow importing eslint-local-rules directories

[BenoitZugmeyer]

This slightly changes the resolveUp logic to consider the module path without extension, to let Node.js resolve it with its own logic. Because Node.js will test for .js extension, we don't need to include it in exts.

This way, using a eslint-local-rules folder containing a index.js file can be used to expose local rules. It is useful when the eslint-local-rules.js file gets a bit big and we need to split it to separate each rule in its own file.

[BenoitZugmeyer]

Thank you for the quick release, I am already using it! 🎉

[cletusw]

Awesome!