* Consider the site's output path in search_for_file
The search_for_file helper function now accepts an optional
output path. If passed, the file will also be searched there.
This is used in the get_url function to search in the
Site::output_path.
In practice, this means cachebust works for files in the
output path.
* Make output_dir required in search_for_file
* Update docs for file searching logic
* Add test for new file searching behavior
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