Proposal: Warn on importing from a filename for which a directory also exists

For example, let's say I have the following files:

```
| something/
|-- index.js
| something.js
| main.js
```

Inside `main.js`, I have the following code:

```js
import { helpfulFunction } from './something';

// do something with helpfulFunction()
```

This is a little dangerous, because did I mean `./something` or `./something/`? I don't think code should be split up like this—in this case, I would take the contents of `something.js` and move them to `something/index.js` and just import from there. A warning would be helpful to track down cases like this.

Start by locating the import-resolution rule entry points and tests in eslint-plugin-import; the issue does not name specific files. Reproduce the example with something.js, something/index.js, and main.js, then determine how a warning should identify the ambiguous import. Done means the plugin warns when both a matching file and directory exist, with coverage for the shown case.

Written by the indexing model from the issue text.