I'm also concerned about automatic tests for other packages. Those import other features to test integrations and they probably do not import using index files. Those could fail if augmentations are not provided.

My biggest doubt regarding this is: why do we have plugin.d.ts and index.ts? Why not merging them into one file? And keeping there all the module augmentation too?

We decided to keep all module augmentation in a separate file (not inlined in index.ts) to be able to include it in the tsconfig.json (link) and tsconfig.release.json (link), so that module augmentation works during the development and build process.

This doesn't have to be applied in the consuming projects as long as they import everything from the main index.ts file, eg.:

// ✔️
import { Table } from '@ckeditor/ckeditor5-table';

// ❌
import Table from '@ckeditor/ckeditor5-table/src/table';

• Let's not expose commands in index.js so far. See one of the above comments.
• Let's not expose stuff like MouseEventsObserver from index.js just to satisfy tests.
• Revert the import-related changes in tests of other packages. Instead, try to change tsconfig.json to use a glob pattern like ../../packages/*/src/augmentation.ts + ../../external/*/packages/*/src/augmentation.ts to make sure that all augmentations are loaded in the test env.

Note: Squash all commits in this branch before the merge so to not pollute the history with all the test import changes.

Adding "./packages/*/src/augmentation.ts" to include array in tsconfig.json didn't work when running testautomated.js. However, it worked when we added "./packages/ckeditor5-table/src/augmentation.ts" to files.

When debugging this with @niegowski and comparing behavior of tsc and testautomated.js, we found that tsc worked with both configurations. After we enabled the logLevel: 'info' option in ts-loader we noticed that it loads tsconfig.js for paragraph plugin, despite testing clipboard plugin in root of the monorepo project. When we added "./packages/*/src/augmentation.ts" to include array in ./packages/ckeditor5-paragraph/tsconfig.json, it started working as expected for clipboard.

We need to investigate why seemingly random tsconfig.json is being loaded by ts-loader. For now, as a quick workaround, we will add "./packages/*/src/augmentation.ts"to include arrays in all packages.

Above issue with the wrong `tsconfig.json` being loaded will be fixed in ckeditor/ckeditor5-dev#847

Note to self: I wanted to comment that you changed formatting of JSON files and I remember that we have a special rule for them in .editorconfig to use two spaces. But then I checked it's for package.jsons only :D And we actually use tabs in other JSON files. Case closed :D

Please update the "Suggested merge commit message" to address changes in this PR.