handling multiple mapping documents in container

[mattgarrish]

epubcheck correctly reports an error that there are two mapping documents in one of the multiple rendition expanded tests (renditions-mapping-multiple-doc-error in the new naming), but then also issues a warning that the first mapping document isn't declared in the OPF manifest.

The multiple renditions specification isn't clear how to prioritize documents in this scenario, but it seems epubcheck only recognizes the second instance and raises OPF-003 for the first instance (i.e., because there are two, report the first as an undeclared file of the package manifest).

I'd expected both to be treated as special container files and only have the error raised. The warning is just confusing.