Webpack 5: plugin don't work if no entry specified in webpack config

[farwayer]

Reproduce demo for this and #347
https://github.com/farwayer/rrwp-issues

[farwayer]

Entry should be updated inside environment hook after all defaults were applied

[pmmmwh]

This is an artificial constraint for now. Updating the entry object anywhere later than the plugin apply phase is potentially too late as we need to ensure the changes propagate to any other code/plugins that observes the entry via hooks.

For Webpack 5 the way to go is #319 - but I currently do not value this as a high priority.

[tfoxy]

Hi @pmmmwh, would it be good to have the entry specified in the readme examples? With a comment explaining that entry is required for webpack 5.

I can do the PR if you want. We can then remove it when #319 is merged.

[pmmmwh]

Feel free to PR. You can either add it in README or TROUBLESHOOTING.