oswrap package is mostly redundant

[nguyen-phillip]

The majority of the oswrap package is thin wrappers around functions from the standard os package, with an extra path-normalization step to handle extended-length paths on Windows. But golang/go#21782 and golang/go#20829 indicate that go now natively handles extended-length paths on Windows. See fixLongPath. So I strongly suspect that most of oswrap (with the possible exception of RemoveOnReboot) is no longer needed.