cli: detect edge-case for MUI icon imports

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/rotten-olives-shop.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Update `config/eslint.js` to forbid imports of `@material-ui/icons/` as well.

packages/cli/config/eslint.js

@@ -80,6 +80,10 @@ module.exports = {
             name: '@material-ui/icons',
             message: "Please import '@material-ui/icons/<Icon>' instead.",
           },
+          {
+            name: '@material-ui/icons/', // because this is possible too ._.
+            message: "Please import '@material-ui/icons/<Icon>' instead.",
+          },
           ...require('module').builtinModules,
         ],
         // Avoid cross-package imports