allow importing eslint-local-rules directories (#6)
allow importing eslint-local-rules directories (#6)

This slightly changes the "resolveUp" logic to consider the module path
without extension, to let Node.js resolve it with its own logic[1].
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.

[1]: https://nodejs.org/api/modules.html#modules_all_together
cletusw/eslint-plugin-local-rules