Pull dynamic extension loading data from the webpack compilation

[jasongrout]

References

Fixes #8842

Code changes

Tweaked the webpack config several ways:

1. Use the new content hashing algorithm and better default module id generation for file names.
2. Embed the information needed to load a dynamic extension in the extension's bundled package.json. This means we can change the build process and the generated build metadata will continue to tell the frontend how to load the extension.
3. Generate a content hash for remoteEntry so it can be cached.

User-facing changes

Hopefully none

Backwards-incompatible changes

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[jasongrout]

@blink1073 - if we could get this into the beta, that would be great.

[jasongrout]

I think with this change, we can cache things in the labextensions handler, i.e., delete this line: https://github.com/jupyterlab/jupyterlab_server/blob/0f8bb54c0badc6570ab2263c78fd1141dc316f9a/jupyterlab_server/handlers.py#L293

[blink1073]

Thanks! Just one question about the log output

[blink1073]

Why remove the log? I've used several times.

[jasongrout]

I'll put in another PR that only saves it in development mode. Does that sound good to you?

[blink1073]

Sounds good. There is a conflict here, I'm afk for the next couple hours but I can rebase and release then.