Return a |PackURI| instance containing the absolute pack URI formed by
        translating *relative_ref* onto *baseURI*.

def from_rel_ref(baseURI, relative_ref):
        """
        Return a |PackURI| instance containing the absolute pack URI formed by
        translating *relative_ref* onto *baseURI*.
        """
        joined_uri = posixpath.join(baseURI, relative_ref)
        abs_uri = posixpath.abspath(joined_uri)
        return PackURI(abs_uri)