Use styleModule in prebuilt extensions

[jasongrout]

References

Fixes #9459

Follow up on #9427

Code changes

In #9427, we introduced a new styleModule key instead of guessing about there being a js file corresponding to a style file. That change did not propagate to prebuilt extensions. This commit fixes prebuilt extensions to use styleModule if available.

Still draft while I modify one of the examples to use styleModule and at least manually check that it is working.

User-facing changes

Backwards-incompatible changes

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[jasongrout]

Good news: this does work (minimally)

Bad news: importing specific modules in a package (e.g., A/style/index.js imports B/style/index.js) does not deduplicate in prebuilt extensions right now. This means that this css deduplication strategy does not work now. More generally, importing a specific module from a shared package will duplicate that module rather than use the shared package version. For example, a prebuilt extension cannot import tokens from mypackage/tokens (since it would duplicate the tokens module, making it useless).

I think we can solve these issues by changing our build system for 3.1—I don't think that any of the changes to make these work requires a breaking change.

Good news 2: likely this is a corner case that is rare among extensions (importing dependency css, or importing from a tokens file or other non-main module in a package, and wanting it to be deduped).

Good news 3: source extensions should be deduped as usual - this only becomes a limitation of prebuilt extensions.

[jasongrout]

Merging since this (a) fixes a bug and (b) makes things consistent, even if it isn't helpful in practice yet.

[jasongrout]

Bad news: importing specific modules in a package (e.g., A/style/index.js imports B/style/index.js) does not deduplicate in prebuilt extensions right now. This means that this css deduplication strategy does not work now. More generally, importing a specific module from a shared package will duplicate that module rather than use the shared package version. For example, a prebuilt extension cannot import tokens from mypackage/tokens (since it would duplicate the tokens module, making it useless).

I experimented with a fix in #9475