Better URI-to-File conversion

This should notably better handle UNC paths, which are often
represented as file URIs with an authority component.

I tested a few I/O-involving steps using [this test file](https://gist.github.com/rdeltour/0bb718ba4ed490be2b77).
There might be some URI-to-File conversion leftovers that are not properly handled (e.g. I haven't touched `XProcConfiguration`), but this PR should cover most of the step implementations.