React-PropTypes-to-prop-types: ignore migrated named import

[simison]

Fixes React-PropTypes-to-prop-types codemod transforming already migrated named imports incorrectly.

These:

import { PropTypes } from 'prop-types';

import { PropTypes as PT } from 'prop-types';

Were incorrectly transformed into this:

import 'prop-types';

This PR adds a check to transform only nodes importing from react, thus ignoring those importing from prop-types.

Previously this codemod would basically assume already migrated stuff to use default export:

import PropTypes from 'prop-types';

Also adds tests for both cases.

Fixes #187

[threepointone]

Thanks for this!