Image flaw checking should be different for translated content

[peterbe]

Part of #3174

This is just one of the cases. But it's an important one, to start with.
Imagine there's an en-US page that contains <img src="foo.png"> in that English page and that works (and that there exists a foo.png next to that English index.html). Now you copy that page to translated-content/files/fr/*. The page should work. No flaw. And the final cheerio transformation, for both the en-US and the fr page, should become: /en-US/docs/Foo/bar/foo.png.

Perhaps a better way to explain it is to look at the test fixtures. We have, in English:

<figure>
  <img src="screenshot.png" alt="Screenshot of colors">
  <figcaption>A perfectly normal image</figcaption>
</figure>

and on the fr equivalent we have:

<figure>
  <img src="screenshot.png" alt="Capture d'écran des couleurs" />
  <figcaption>Une image parfaitement normale</figcaption>
</figure>

What you get in the end is:

[peterbe]

My intention is to break down this work into multiple PRs. The next thing that should work is if the French translation had:

<figure>
  <img src="/en-US/docs/Web/Foo/screenshot.png" alt="Capture d'écran des couleurs" />
  <figcaption>Une image parfaitement normale</figcaption>
</figure>

which should also work. But one thing at a time.

[fiji-flo]

👍 one step at a time!