Ensure compiling hast doesn't fail if path contains export

[zephraph]

Okay, so this one threw me for a hell of a loop.

I was writing a test for another plugin that wraps mdx loader and it kept inexplicably failing. I went as far as copying a passing test from somewhere else and it was still failing.

The error was pretty mysterious too:

ReferenceError: s is not defined

      292 | 
      293 | const layoutProps = {
    > 294 |   s
          |   ^
      295 | };
      296 | const MDXLayout = docs_page({
      297 |   layout: 'docs-page',

Turns out, in mdx-hast-to-jsx.js, the calculation of exportNames will fail if there is a directory named export in the exported file path. Since my test directory was called layouts-exports-ssg it caused layoutProps to output garbage because the regex had a miss-match.

You can check the regex here: https://regexr.com/4sar8

[vercel]

This pull request is being automatically deployed with ZEIT Now (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://zeit.co/mdx/mdx/6luar10wj
✅ Preview: https://mdx-git-fork-zephraph-fix-mysterious-regex-crash.mdx.now.sh

[johno]

Oh, whoa! Thanks for tracking this down and providing a fix <3.