Fix babel config file detection

[jdreesen]

Since v7 Babel supports two types of config formats:

• Project-wide configuration
  • babel.config files, with the different extensions (json, js, cjs, mjs)
• File-relative configuration
  • .babelrc files, with the different extensions (none, json, js, cjs, mjs)
  • package.json files with a "babel" key

Encore, however, only checks for the existence of file-relative config files.

This is a problem if you want to compile packages from the node_modules folder (via .configureBabel(null, {includeNodeModules: ['...']})) because project-wide configuration files are needed for this.

This means that if you only have a .babelrc.js file, the package from the node_modules folder will be compiled by Babel, but without the settings defined in the .babelrc.js file.

If you now rename the .babelrc.js to babel.config.js, Encore fails to detect it and applies its default Babel config, which results in everything being complied without the settings defined in babel.config.js.

Note: Configuring babel through Encore is not an option for me, because I want to make use of Babels Config Function API which is not available in Encore afaik.
And apart from that I prefer to have the configuration in dedicated files.

So my solution for now is to keep an empty .babelrc.js to make Encore think there is a Babel config file, and have an additional babel.config.js file with the real config. This results in both the application code and node modules being compiled with the desired settings.

---

But of course I dug a little deeper into Encore and Babel to fix the problem and the fix is using the hasFilesystemConfig() method instead of checking the babelrc property (the wording in the comment is a bit outdated, I already created a PR for that ;)):
https://github.com/babel/babel/blob/af669297efd775016725ee39318cdb391cf00a21/packages/babel-core/src/config/partial.js#L191-L200

I added some tests, too. I'm not sure if it's worth it to add tests for the different config file extensions, though.

I tried to improve the error messages as well. Tell me if you want another wording or different file examples and I'll change it.

[jdreesen]

This may be related to #558 and #670 (comment).

[weaverryan]

Wow! This is an incredible PR. I can see your comment fix on babel was already merged. You made minimal changes to Encore, they make sense, and you added tests. One of my favorite PR's :).

[weaverryan]

Thank you Jacob!

[ankurk91]

We could use cosmicconfig here, example:

const cosmiconfig = require('cosmiconfig');

const userConfigExists = () => {
  return !!cosmiconfig('babel', {
    searchPlaces: [
      'package.json',
      '.babelrc',
      '.babelrc.js',
      'babel.config.js'
    ]
  }).searchSync()
};

This package can used to search for postcss configs as well.

[Kocal]

That's a quite solution, but I think it's better to let the package find its own configuration itself. This way we don't have to maintain a list of search places.