Add path package

For testing purposes we need to get the long path of filesystem paths
on Windows machines. By default non-UTF8 syscalls will return the short path
(Ex:"C:\PROGRA~1" instead of "C:\Program Files"). This package exports
previously private functions used in the symlink package to determine the
long path of a file/directory.
This isn't a great name, as it clashes with the oft-used stdlib path package.
Since it provides windows-only functionality, perhaps it should be named "windowspath",
or "longpath" ?

I'd also like to know where these short paths are coming from, and
hence why we actually need this package. In general I'm not
keen on OS-specific packages, preferring to define OS-independent
abstractions (perhaps with system-dependent parts) instead.

One possibility is to define GetLongPath under non-windows platforms
as a no-op; but it still raises the question of when we'd need to call it.