Declare Webpack loaders with require.resolve()

[tibdex]

Fix #15298 by ensuring that loaders are resolved from @jupyterlab/builder instead of from the package being built.

This will allow to get rid of css-loader and style-loader in the devDependencies of extension-cookiecutter-ts.

By the way, worker-loader seems unused. Can we remove it?

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[fcollonval]

Thanks @tibdex

Regarding worker-loader I'm fine with its removal. But I'ld like to be cautious and to do that in a separate PR that will target the next minor version (ETA December). Would you have time to open that PR?

[tibdex]

For worker-loader, even though it's not used directly in this repo, it looks like some extensions still rely on it: #10033 so I guess it's safer to keep it.

[krassowski]

worker-loader is deprecated for webpack 5 as it supports native syntax for webworkers. Neither works well with module federation (#10197 (comment)). Also, extensions can extend the webpack configuration with custom loaders so they would still be able to use worker-loader if they choose so. But we should certainly drop it no later than for JupyterLab 5.0.

[tibdex]

The failed CI jobs don't seem related to my changes. Is there anything left I can do to help merge this?

[krassowski]

The only thing missing is deciding a milestone (4.0.x or 4.1) I am not certain how much of a potential to break things these changes have. Do you think it is safe enough change to backport it to 4.0.x or should it or would it be better for extension authors to only get it in 4.1?

While failures are indeed unrelated I restarted some tests to clear the statuses

[tibdex]

I'm pretty sure this won't cause any regressions and can thus be safely backported to 4.0 but I don't mind waiting for 4.1 to get it.

[krassowski]

@meeseeksdev please backport to 4.0.x