Combine createElement and JSX modules

There's a ton of overlap between the createElement implementation and
the JSX implementation, so I combined them into a single module.

In the actual build output, the shared code between JSX and
createElement will get duplicated anyway, because react/jsx-runtime and
react (where createElement livs) are separate, flat build artifacts.

So this is more about code organization — with a few key exceptions,
the implementations of createElement and jsx are highly coupled.

packages/react/src/ReactChildren.js

@@ -24,7 +24,7 @@ import {
 } from 'shared/ReactSymbols';
 import {checkKeyStringCoercion} from 'shared/CheckStringCoercion';
 
-import {isValidElement, cloneAndReplaceKey} from './ReactElement';
+import {isValidElement, cloneAndReplaceKey} from './jsx/ReactJSXElement';
 
 const SEPARATOR = '.';
 const SUBSEPARATOR = ':';

packages/react/src/ReactClient.js

@@ -30,7 +30,7 @@ import {
   createFactory,
   cloneElement,
   isValidElement,
-} from './ReactElement';
+} from './jsx/ReactJSXElement';
 import {createContext} from './ReactContext';
 import {lazy} from './ReactLazy';
 import {forwardRef} from './ReactForwardRef';