More robust loading of CJS plugins using require()

[bmish]

For CJS plugins, we should just load them with Node's require function. This defers to Node on resolving/loading the package entry point, meaning we don't have to try to handle any of that logic ourselves. As a result, support for CJS plugins should be improved. This greater robustness means any file types (e.g. JSON) handled by require are also supported now.

Only for ESM plugins do we need to manually try to resolve the package entry point to pass to the import function. That's because you can't just do import('path/to/plugin') for importing a local directory. import does not automatically resolve anything, so we have to do attempt to do it ourselves by looking at exports in package.json.

Related to #140. Helps with #132.

[ljharb]

It seems like it'd be easier to just author in CJS modules, and require or await import() as needed?

[bmish]

It seems like it'd be easier to just author in CJS modules, and require or await import() as needed?

@ljharb just to respond to this:

1. I have the change working. The only problem is that I haven't gotten it to work in the jest tests yet. Making slow progress on the tests dealing with require weirdness in jest.
2. Authoring this tool in CJS wouldn't help and would actually preclude us from being able to import() ESM plugins (which is essential since the new ESLint config system supports ESM configs).

[ljharb]

Why? import(), whether in CJS or ESM, always loads ESM.

[bmish]

Oh, nice, I think I overlooked how dynamic import can load ESM from CJS. Either way, it's nicer to write code in ESM.

[ljharb]

That seems like something that could be addressed with babel without adding the complexity of createRequire, but that's certainly up to you.