When xml provides source paths we should use them

Attempting to address: https://github.com/edx/diff-cover/issues/113

I have some concerns just merging this. Mainly in that my sample size of cobertura files is pretty small. This solves the specific case in the issue and seems to be ok with the java projects ive looked at it would be nice if we could find more to validate this with.

Also, _get_src_path_line_nodes is starting to get a bit ... wild. This may be addressed by using a specific cobertura parser like https://github.com/SMFOSS/pycobertura

Or perhaps simply some refactoring is in order