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.
The shortpaths come from syscalls, and are mostly coming from gocheck... we could just fix gocheck, but there's still going to be windows calls that will need this (like the symlink stuff).  

Ideally the longpath stuff would be abstracted away to the windows-specific implementations of other things (like symlink), and not need to pollute our common code with function calls that are no-ops on some platforms.