Fix circular self-imports and add no-self-package-imports lint rule

- Fixes the `Cannot access '_AppRootElementBlueprintesm' before
  initialization` crash in `@backstage/frontend-plugin-api` caused by a
  self-referential import in the packaged ESM.
- Cleans up similar self-imports in `@backstage/catalog-model`,
  `@backstage/core-plugin-api`, `@backstage/plugin-catalog-node`,
  `@backstage/plugin-kubernetes-common`, and
  `@backstage/plugin-kubernetes-node`. Value imports switch to relative
  paths; type-only imports use `import type` so they're erased at
  runtime.
- Adds a new `@backstage/no-self-package-imports` ESLint rule. It reads
  each package's `exports` map, traverses the relative import graph from
  every entry's source file, and only reports imports where the current
  file is in the same bundle as the target entry (same-entry). Files
  that aren't reachable from any entry (tests, scripts, orphans) are
  skipped. `import type`, `package.json` imports, and cross-entry
  self-imports are allowed by default; cross-entry can be opted into
  with `allowCrossEntry: false`.

Signed-off-by: Marat Dyatko <maratd@spotify.com>
Made-with: Cursor

packages/frontend-plugin-api/src/blueprints/AppRootElementBlueprint.tsx

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { ExtensionBoundary } from '@backstage/frontend-plugin-api';
+import { ExtensionBoundary } from '../components/ExtensionBoundary';
 import { coreExtensionData, createExtensionBlueprint } from '../wiring';
 
 /**