Homogenize path-separator in staging_parse

The staging_parse() function manipulates input using Ruby libraries and path logic. However, the Ruby URI library expects that (even on Windows) paths will use "/" as a separator. Rather than special-case everything, resolve this inconsistency regarding Windows paths by enforcing that backslashes will simply be converted to forward-slashes before trying to operate on them with staging_parse(). Otherwise, using a source or target in the form of a Windows path with backslashes will cause the staging_parse() function to abort.