[code-infra] Babel plugin to fully resolve imported paths

[Janpot]

Working on #43264 I found that we can make rollup work quite well for us to resolve modules, except that they inline interop helpers that we can't seem to dedupe the way we can in babel.

This PR solves the problem differently with a custom babel plugin that can resolve module imports to their actual emitted file.

This PR changes imports in the build output from

// packages/mui-material/build/index.js
export * from './Accordion';

// packages/mui-material/build/Breadcrumbs/BreadcrumbCollapsed.js
import MoreHorizIcon from '../internal/svg-icons/MoreHoriz';

to

// packages/mui-material/build/index.js
export * from './Accordion/index.js';

// packages/mui-material/build/Breadcrumbs/BreadcrumbCollapsed.js
import MoreHorizIcon from '../internal/svg-icons/MoreHoriz.js';

It resolves those imports to the full filename with extension.

See https://nodejs.org/docs/v20.16.0/api/esm.html#mandatory-file-extensions

A file extension must be provided when using the import keyword to resolve relative or absolute specifiers. Directory indexes (e.g. './startup/index.js') must also be fully specified.

This behavior matches how import behaves in browser environments, assuming a typically configured server.

This doesn't update imports for the icons package, as these modules are pre-baked in the repository. We can run pnpm --filter @mui/icons-material build:lib to update the icons as part of this PR or separately.

I'm also adding some type checking to the babel.config.js

Corresponding PR on X

---

Noticed this problem while testing out the changes.

[mui-bot]

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https://deploy-preview-43294--material-ui.netlify.app/

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[Janpot]

Have been testing this locally from csbci published packages and can't seem to break it

[michaldudak]

The implementation looks good, but could you place the plugin somewhere we can package and distribute with npm (internal-scripts, perhaps)?

[Janpot]

Done. Added a @mui/internal-babel-plugin-resolve-imports package

edit: added it to csbci as well

[michaldudak]

Thanks!

[michaldudak]

That's unusual initial version ;)

[Janpot]

😄 I copied the package.json of @mui/internal-scripts and didn't give it any further thought.