Feature: Allow exports to use sub-folder as a name

[cawfeecoder]

Description

When using the --exports flag, I would like the sub-folder to name the export as opposed to the filename

For example, I have the following structure:

.
├── src              
├────  button
├──────  index.js

Currently, using the --exports flag will name one of the exports as index instead of button. It would be nice if the index.js is inside a sub-folder to be able to use that as the name so the generated exports in package.json appear as:

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

vs

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

[UpperCod]

I like the idea, let's complete the PR to perform the merge