func (l Label) Rel(repo, pkg string) Label {
	if l.Relative || l.Repo != repo {
		return l
	}
	if l.Pkg == pkg {
		return Label{Name: l.Name, Relative: true}
	}
	return Label{Pkg: l.Pkg, Name: l.Name}
}
func (l Label) Rel(repo, pkg string) Label {
	if l.Relative || l.Repo != repo {
		return l
	}
	if l.Pkg == pkg {
		return Label{Name: l.Name, Relative: true}
	}
	return Label{Pkg: l.Pkg, Name: l.Name}
}
// Rel attempts to compute a relative label from this label. If this label
// is already relative or is in a different package, this label may be
// returned unchanged.