Split buildutils into buildutils and builder

[jasongrout]

References

Fixes #8857

Code changes

The new builder package is used to build extensions. The buildutils now contains repo-specific utilities.

User-facing changes

Cleaner package for building and less dependencies consumed for people wanting to use our build utilities like update:dependency.

Backwards-incompatible changes

Moves build out of buildutils and into builder.

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[jasongrout]

@blink1073 - here is at least a start at splitting off the builder package. Feel free to take over if you'd like.

[blink1073]

Thanks, picking this up now

[blink1073]

Usage will fail until we cut another release because @jupyterlab/builder is not in jupyterlab/staging/package.json dependencies.

[blink1073]

I released alpha 12

[jasongrout]

Thanks!