Added Material UI 4 import rule to package @backstage/frontend-app-api

Signed-off-by: Juan Pablo Garcia Ripa <sarabadu@gmail.com>

packages/frontend-app-api/.eslintrc.js

@@ -1 +1,5 @@
-module.exports = require('@backstage/cli/config/eslint-factory')(__dirname);
+module.exports = require('@backstage/cli/config/eslint-factory')(__dirname, {
+  rules: {
+    '@backstage/no-top-level-material-ui-4-imports': 'error',
+  },
+});

packages/frontend-app-api/src/extensions/AppNav.tsx

@@ -23,7 +23,7 @@ import {
   createNavItemExtension,
   createNavLogoExtension,
 } from '@backstage/frontend-plugin-api';
-import { makeStyles } from '@material-ui/core';
+import { makeStyles } from '@material-ui/core/styles';
 import {
   Sidebar,
   useSidebarOpenState,

packages/frontend-app-api/src/extensions/components.tsx

@@ -16,7 +16,7 @@
 
 import React from 'react';
 // TODO: Dependency on MUI should be removed from core packages
-import { Button } from '@material-ui/core';
+import Button from '@material-ui/core/Button';
 
 import {
   createComponentExtension,