Remove barrel files

[swissspidy]

Summary

See #13618 for context.

• [x] Add ESLint rule to detect barrel files
• [ ] Eliminate some or all barrel files (maybe only in 1-2 packages for now)
• [ ] Measure difference before/after change

User-facing changes

N/A

Testing Instructions

• [x] This is a non-user-facing change and requires no QA

Reviews

Does this PR have a security-related impact?

No

Does this PR change what data or activity we track or use?

No

Does this PR have a legal-related impact?

No

Checklist

• [x] This PR addresses an existing issue and I have linked this PR to it
• [x] I have tested this code to the best of my abilities
• [x] I have verified accessibility to the best of my abilities (docs)
• [x] I have verified i18n and l10n (translation, right-to-left layout) to the best of my abilities
• [x] This code is covered by automated tests (unit, integration, and/or e2e) to verify it works as intended (docs)
• [x] I have added documentation where necessary
• [x] I have added a matching Type: XYZ label to the PR

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Fixes #13618