[DatePicker] Use only path imports from @material-ui/core

Material-UI forbid imports over two levels as it can lead to duplication in people bundles. Meaning, if somebody does:

import { Modal } from '@material-ui/core'; // 1.
import TrapFocus from '@material-ui/core/Modal/TrapFocus'; // 2.

It will bundle 1. and 2.:

1. https://unpkg.com/browse/@material-ui/core@4.9.10/esm/Modal/TrapFocus.js
2. https://unpkg.com/browse/@material-ui/core@4.9.10/Modal/TrapFocus.js

Now, given we do:

https://github.com/mui-org/material-ui-pickers/blob/7bed283699ef768936a3ec7c5dc89571492dddd4/lib/src/wrappers/DesktopPopperWrapper.tsx#L6

in the codebase, we very likely cause this double bundling quite often. For instance:

import { Dialog } from '@material-ui/core';
import { DateRangePicker } from '@material-ui/pickers';

[TrySound]

@dmtrKovalenko @material-ui/core/Modal/TrapFocus.js and @material-ui/core/esm/Modal/TrapFocus.js is not the same file

[mbrookes]

😱(Has it been that long?!)

Could you extend the number of people that have the publish rights

Done.