[core] Flatten the import path

[oliviertassinari]

This pull-request was time-consuming, so many lite changes to make 🤖. Closes #9532

Breaking change

Motivation

1. It's a simple pattern to learn. People won't need to go back and forth with the documentation to learn the import paths 💭.
2. People bundle size will decrease 🚀.
3. In an ideal world, we would import everything from the root module and tree sharking would be taken care of for us. This change doesn't matter in this world ☮️.

import {
  Table,
  TableBody,
  TableCell,
  TableFooter,
  TablePagination,
  TableRow,
} from 'material-ui';

The diff

-import CircularProgress from '@material-ui/core/Progress/CircularProgress';
+import CircularProgress from '@material-ui/core/CircularProgress';

-import { ListItem } from '@material-ui/core/List';
+import ListItem from '@material-ui/core/ListItem';

Upgrade path

We provide a codemod to automate the migration: https://github.com/mui-org/material-ui/tree/master/packages/material-ui-codemod#import-path. I have used it to upgrade all the demos in the documentation :).

[koutsenko]

codemod fails when using experimental features in src like export from.

[jjwilliams42]

The link you provided is a 404

[oliviertassinari]

@jackjwilliams I have updated the link.