fix(gatsby-plugin-netlify-cms): react18-compatible require resolve

[kspeyanski]

Description

Hello,

Currently with both latest and next versions of gatsby-plugin-netlify-cms are not fully compatible with react@18, and throw an error. Also reproducible with gatsby-starter-netlify-cms if upgraded to react@18:

Package subpath './umd/react.production.min.js' is not defined

The issue seems to be related to how the plugin is utilizing the WebpackCopyPlugin to copy the umd distribution of react and react-dom:

gatsby/packages/gatsby-plugin-netlify-cms/src/gatsby-node.js

Lines 210 to 222 in 0809c20

	new CopyPlugin({
	patterns: externals.flatMap(
	({ name, assetName, sourceMap, assetDir }) =>
	[
	{
	from: require.resolve(path.join(name, assetDir, assetName)),
	to: assetName,
	},
	sourceMap && {
	from: require.resolve(path.join(name, assetDir, sourceMap)),
	to: sourceMap,
	},
	].filter(Boolean)

With version 18 of react and react-dom the exports field has been added, which unfortunately does not include any of the umd distributions:
https://github.com/facebook/react/blob/e8f4a6653dc3f5d9702236a3abed887ca44dcfbb/packages/react/package.json#L22

Reading between the lines of the NodeJS Resolution Algorithm it seems only the defined subpath in "exports" can be imported by a consumer - thus the error.

Proposal

For this, I can propose always require.resolve-ing the package.json and navigate to the umd scripts from there. This seems somewhat future-proof due to the following discussion, where from my understanding, the overall desire is to always allow require.resolve of the package.json file, regardless of its entry in the "exports" field:
nodejs/node#33460

Related Issues

Fixes: #35364

[pieh]

Thank you! Change makes total sense

[i-am-the-slime]

Is this released in any way? Can I use it? I have this error

[chocobuckle]

This fix included in the latest release? I tried installing v6.12.1 but still getting the same error.

[LekoArts]

It'll be released today, new minor releases are happening every two weeks. If it is published in a patch version, the PR will receive a comment.