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/eslint-plugin/index.js

@@ -24,6 +24,7 @@ module.exports = {
         '@backstage/no-undeclared-imports': 'error',
         '@backstage/no-mixed-plugin-imports': 'warn',
         '@backstage/no-ui-css-imports-in-non-frontend': 'error',
+        '@backstage/no-self-package-imports': 'error',
       },
     },
   },
@@ -34,5 +35,6 @@ module.exports = {
     'no-top-level-material-ui-4-imports': require('./rules/no-top-level-material-ui-4-imports'),
     'no-mixed-plugin-imports': require('./rules/no-mixed-plugin-imports'),
     'no-ui-css-imports-in-non-frontend': require('./rules/no-ui-css-imports-in-non-frontend'),
+    'no-self-package-imports': require('./rules/no-self-package-imports'),
   },
 };