Hoist current file name for transform-react-jsx-source

For better tooling support it's important to include absolute file name for JSX
elements. However, having them inline will inflate resulting file size, so we
move the file name to a constant declared at the beginning of the file and
reference it from `__source` attribute.

packages/babel-plugin-transform-react-jsx-source/test/fixtures/react-source/basic-sample/expected.js

@@ -1,4 +1,5 @@
+var __jsxFileName = "/I/am/not/sure/how/to/get/path/to/test/fixtures/react-source/basic-sample/actual.js";
 var x = <sometag __source={{
-  fileName: "../test/fixtures/react-source/basic-sample/actual.js",
+  fileName: __jsxFileName,
   lineNumber: 1
-}} />;
+}} />;