In this diff I'm making a step forward to esm support.

Here I export scoped variables instead of object. Conditions are
resolved with with ternary operator.

To solve the problem of using both `import * as React from 'react'`
and `import React from 'react'` I removed `__esModule` field with simple
regexp. In the future rollup release this will be achived with
[new rollup option](rollup/rollup#2287).

New bundles are tested with webpack, rollup and babel-jest. All ways
produce same results.

There was a problem with reexports in #11526
and as you can see in examples below it's solved so we can migrate
react-dom to named exports aswell in the next PR.

before

```js
var React = {
  Children: {
    map: mapChildren,
    forEach: forEachChildren,
    count: countChildren,
    toArray: toArray,
    only: onlyChild
  },
  createRef: createRef,
  Component: Component,
  PureComponent: PureComponent,
  createContext: createContext,
  forwardRef: forwardRef,
  Fragment: REACT_FRAGMENT_TYPE,
  StrictMode: REACT_STRICT_MODE_TYPE,
  unstable_AsyncMode: REACT_ASYNC_MODE_TYPE,
  unstable_Profiler: REACT_PROFILER_TYPE,
  createElement: createElementWithValidation,
  cloneElement: cloneElementWithValidation,
  createFactory: createFactoryWithValidation,
  isValidElement: isValidElement,
  version: ReactVersion,
  __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: {
    ReactCurrentOwner: ReactCurrentOwner,
    // Used by renderers to avoid bundling object-assign twice in UMD bundles:
    assign: _assign
  }
};
if (enableSuspense) {
  React.Timeout = REACT_TIMEOUT_TYPE;
}
{
  _assign(React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED, {
    // These should not be included in production.
    ReactDebugCurrentFrame: ReactDebugCurrentFrame,
    // Shim for React DOM 16.0.0 which still destructured (but not used) this.
    // TODO: remove in React 17.0.
    ReactComponentTreeHook: {}
  });
}
var React$2 = Object.freeze({
	default: React
});
var React$3 = ( React$2 && React ) || React$2;
// TODO: decide on the top-level export form.
// This is hacky but makes it work with both Rollup and Jest.
var react = React$3.default ? React$3.default : React$3;
module.exports = react;
```

after

```js
exports.Children = Children;
exports.createRef = createRef;
exports.Component = Component;
exports.PureComponent = PureComponent;
exports.createContext = createContext;
exports.forwardRef = forwardRef;
exports.isValidElement = isValidElement;
exports.Fragment = Fragment;
exports.StrictMode = StrictMode;
exports.unstable_AsyncMode = unstable_AsyncMode;
exports.unstable_Profiler = unstable_Profiler;
exports.createElement = createElement$$1;
exports.cloneElement = cloneElement$$1;
exports.createFactory = createFactory$$1;
exports.version = version;
exports.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED = __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;
exports.Timeout = Timeout;
```