require bugfixed Archive::Tar::Wrapper
allow "already assign to VERSION" to be fatal
install with cpanm by default (for now)
PkgVersion: Don't attempt to save if file unchanged

PkgVersion always attempted to save the file, regardless whether
it had changed. Now, we track whether the file was successfully
munged, and only save if so. This is an optimization for the
case where no changes are made.
since we recommend the user installs this, do it for them if available