feat: eslint rule to check forbidden plugin imports

basically verify-local-dependencies.js but done during linting also in
the 3rd party repositories.

Signed-off-by: Hellgren Heikki <heikki.hellgren@op.fi>

packages/eslint-plugin/index.js

@@ -22,6 +22,7 @@ module.exports = {
         '@backstage/no-forbidden-package-imports': 'error',
         '@backstage/no-relative-monorepo-imports': 'error',
         '@backstage/no-undeclared-imports': 'error',
+        '@backstage/no-mixed-plugin-imports': 'error',
       },
     },
   },
@@ -30,5 +31,6 @@ module.exports = {
     'no-relative-monorepo-imports': require('./rules/no-relative-monorepo-imports'),
     'no-undeclared-imports': require('./rules/no-undeclared-imports'),
     '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'),
   },
 };