Add ELSint rule to check inter-packages imports

[jodator]

📝 Provide a description of the new feature

Follow-up for the #8395. We're going to change how we import things from other packages.

This ESLint rule should check if the imports from other packages are pointing to the package entry file instead of direct import.

In other words, depending on how the #8578 is resolved the imports should point to entry file:

import { Plugin } from '@ckeditor/ckeditor5-core/core';
// or (see #8578)
import { Plugin } from 'ckeditor/core';

The wrong import would be:

import Plugin from '@ckeditor/ckeditor5-core/src/plugin';

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[Reinmar]

Verify that the only thing that a package has in dependencies is ckeditor5 (from the set of pkgs maintained by us). Second rule – only import from the current package or ckeditor5 (also, only applies to the set of our packages). Let's have ESLint rules for that (#8581). Exception: the ckeditor5 package.

[Reinmar]

TODO: Update remaining incorrect imports (also in CF and internal).

[pomek]

In the Title plugin we need to refactor the code a bit:

-import Plugin from '@ckeditor/ckeditor5-core/src/plugin';
-import Paragraph from '@ckeditor/ckeditor5-paragraph/src/paragraph';
+import { Plugin } from 'ckeditor5/src/core';
 
-import ViewDocumentFragment from '@ckeditor/ckeditor5-engine/src/view/documentfragment';
-import ViewDowncastWriter from '@ckeditor/ckeditor5-engine/src/view/downcastwriter';
-import first from '@ckeditor/ckeditor5-utils/src/first';
+import { first } from 'ckeditor5/src/utils';
 import {
+	UpcastWriter,
 	needsPlaceholder,
 	showPlaceholder,
 	hidePlaceholder,
 	enablePlaceholder
-} from '@ckeditor/ckeditor5-engine/src/view/placeholder';
+} from 'ckeditor5/src/engine';

@@ -44,7 +42,7 @@ export default class Title extends Plugin {
 	 * @inheritDoc
 	 */
 	static get requires() {
-		return [ Paragraph ];
+		return [ 'Paragraph' ];
 	}

 	/**
@@ -149,10 +147,11 @@ export default class Title extends Plugin {
 		const data = editor.data;
 		const model = editor.model;
 		const root = editor.model.document.getRoot();
-		const viewWriter = new ViewDowncastWriter( editor.editing.view.document );
+		const view = editor.editing.view;
+		const viewWriter = view.change( viewWriter => viewWriter );
 
 		const rootRange = model.createRangeIn( root );
-		const viewDocumentFragment = new ViewDocumentFragment( editor.editing.view.document );
+		const viewDocumentFragment = new UpcastWriter( view.document ).createDocumentFragment();
 
 		data.downcastDispatcher.conversionApi.options = options;

Are we fine with the changes above?