[WIP] Add transform to reset modules in tests

[rickhanlonii]

[WIP] Overview

Adds a transform that will transform imports like:

const React = require('react');

to this:

let React;
beforeEach(() => {
  React = require('react');
});

Combined with the last change to run jest.resetModules() between each test, this will reset the React/ReactDOM/etc import for each test. This means we can codemod the ugly manually way of doing this now, but it can result in some strange test failures.

[react-sizebot]

Comparing: a1ace9d...7d0b6a5

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	176.66 kB	176.66 kB	=	55.01 kB	55.01 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	178.65 kB	178.65 kB	=	55.59 kB	55.59 kB
facebook-www/ReactDOM-prod.classic.js	=	591.78 kB	591.78 kB	=	104.44 kB	104.44 kB
facebook-www/ReactDOM-prod.modern.js	=	575.53 kB	575.53 kB	=	101.54 kB	101.54 kB
test_utils/ReactAllWarnings.js	Deleted	67.02 kB	0.00 kB	Deleted	16.42 kB	0.00 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
test_utils/ReactAllWarnings.js	Deleted	67.02 kB	0.00 kB	Deleted	16.42 kB	0.00 kB

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