Verify the dependency checker

[psmyrek]

Provide a description of the task

The dependency checker allows to import ComponentFactory from '@ckeditor/ckeditor5-ui/src/componentfactory'; in ckeditor5-core package (packages/ckeditor5-core/src/editor/editorui.js), despite lack of @ckeditor/ckeditor5-ui in ckeditor5-core's dependencies. Verify why.

Additionally:

• check that it would tell you, that your DLL package must have ckeditor5 in the dependencies, if you imported something from ckeditor5/....
• check that it would tell you, that your non-DLL package has @ckeditor/ckeditor5-* in the dependencies.
• add it to husky so it's run locally on commit.
• take the occasion to rethink this tool's logic (check out with @ma2ciek why do they have a separate dependency checker in CF - maybe it's a good moment to unify them).

[psmyrek]

The dependency checker verifies, if a package included in the code is missing in package.json, but it currently does not verify, if a package already listed as dependencies or devDependencies should belong to that list: i.e. all packages in devDependencies should be there if they are used only in tests, but if any of it is imported in the src/**/*.js then this particular package should be defined in dependencies. This is the answer for the question, why the dependency checker does not complain about importing something from @ckeditor/ckeditor5-ui in ckeditor5-core despite lack of @ckeditor/ckeditor5-ui in ckeditor5-core's dependencies - it is actually included in devDependencies there!

So I'm adding another point to improve:

• verify that all packages belong to proper group: dependencies or devDependencies

[psmyrek]

The dependency checker does not inform about unused packages in dependencies or devDependencies, so:

• fix detecting unused packages

[psmyrek]

The dependency checker does not check CSS files properly, so:

• fix handling CSS files