Summary:
Setting the module extension to .tsx overrode the defaults, meaning .js files were no longer being checked when imported.
This adds the defaults back to the extensions to be checked, including `.js`
It does seem to cause errors importing `.tsx` at the moment, but there aren't many so I've just $FlowFixMe'd them for now.
https://flow.org/en/docs/config/options/#toc-module-file-ext-string
Reviewed By: passy
Differential Revision: D16333800
fbshipit-source-id: 7bea92c038048234b4f634704f71c15d79ab3c63