wip: Attempt at making cargo working on whole project as documented

(Make cargo work on whole project as documented)

According to `:help ale-integration-rust`, using cargo as a linter should
check the entire project. It appears this was the case initially, but it
broke somewhere along the way.

Setting `filename` to the output in the handler and avoid filtering the
spans on `file_name` goes a long way in restoring the intended behavior.

However test cases fail, even after attempting to update them. Please
see next commit.

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