oswrap package is mostly redundant

The majority of the [oswrap](https://github.com/google/googet/tree/master/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 https://github.com/golang/go/issues/21782 and https://github.com/golang/go/issues/20829 indicate that go now natively handles extended-length paths on Windows. See [fixLongPath](https://github.com/golang/go/blob/598df45fceb6e10d643ed0c07a3df80cffd507f1/src/os/path_windows.go#L50). So I strongly suspect that most of oswrap (with the possible exception of RemoveOnReboot) is no longer needed.