Review NormalizePathSeparators for source files

We currently normalize directory separators according to the OS we're running on, excluding paths that start with `http(s):` or paths that are deterministic (`/_/` , `/_1/`) etc.

There are a couple of other possible path prefixes like `file:`, and especially on Windows stuff like `\\?\` or `\\.\`. Don't think they will appear in reports, but who knows.