Paths are still passed around as relative. Code which needs to read files
(images, templates) and code which needs to manipulate paths
(Jekyll::ResponsiveImage::Utils#image_hash) has to be aware of the site source
path.
This got a bit messy in the end, but I couldn't find a better way without doing
a big refactor of the entire plugin.
* Move 'fixtures' to 'test-site' because, like, that's what it is
* Refactor to make reasoning about file paths easier
* Write a failing test
* Conventions, conventions
* Always treat paths as relative to the Jekyll site source