Updated MUI imports for core-components

Signed-off-by: Andre Wanlin <awanlin@spotify.com>

packages/core-components/.eslintrc.js

@@ -1,6 +1,7 @@
 module.exports = require('@backstage/cli/config/eslint-factory')(__dirname, {
   rules: {
     'jest/expect-expect': 0,
+    '@backstage/no-top-level-material-ui-4-imports': 'error',
   },
   restrictedImports: [
     {

packages/core-components/src/components/AlertDisplay/AlertDisplay.tsx

@@ -19,7 +19,7 @@ import IconButton from '@material-ui/core/IconButton';
 import Snackbar from '@material-ui/core/Snackbar';
 import Typography from '@material-ui/core/Typography';
 import CloseIcon from '@material-ui/icons/Close';
-import { Alert } from '@material-ui/lab';
+import Alert from '@material-ui/lab/Alert';
 import React, { useEffect, useState } from 'react';
 import { coreComponentsTranslationRef } from '../../translation';