Combine ReactJSXElementValidator with main module (facebook#28317)

There are too many layers to the JSX runtime implementation. I think
basically everything should be implemented in a single file, so that's
what I'm going to do.

As a first step, this deletes ReactJSXElementValidator and moves all the
code into ReactJSXElement. I can already see how this will help us
remove more indirections in the future.

Next I'm going to do start moving the `createElement` runtime into this
module as well, since there's a lot of duplicated code.

packages/react/src/jsx/ReactJSX.js

@@ -8,15 +8,21 @@
  */
 import {REACT_FRAGMENT_TYPE} from 'shared/ReactSymbols';
 import {
-  jsxWithValidationStatic,
-  jsxWithValidationDynamic,
-  jsxWithValidation,
-} from './ReactJSXElementValidator';
-import {jsx as jsxProd} from './ReactJSXElement';
-const jsx: any = __DEV__ ? jsxWithValidationDynamic : jsxProd;
+  jsxProd,
+  jsxProdSignatureRunningInDevWithDynamicChildren,
+  jsxProdSignatureRunningInDevWithStaticChildren,
+  jsxDEV as _jsxDEV,
+} from './ReactJSXElement';
+
+const jsx: any = __DEV__
+  ? jsxProdSignatureRunningInDevWithDynamicChildren
+  : jsxProd;
 // we may want to special case jsxs internally to take advantage of static children.
 // for now we can ship identical prod functions
-const jsxs: any = __DEV__ ? jsxWithValidationStatic : jsxProd;
-const jsxDEV: any = __DEV__ ? jsxWithValidation : undefined;
+const jsxs: any = __DEV__
+  ? jsxProdSignatureRunningInDevWithStaticChildren
+  : jsxProd;
+
+const jsxDEV: any = __DEV__ ? _jsxDEV : undefined;
 
 export {REACT_FRAGMENT_TYPE as Fragment, jsx, jsxs, jsxDEV};