'Return a list of |_SerializedPart| instances corresponding to the
parts in *phys_reader* accessible by walking the relationship graph
starting with *pkg_srels*.'

@staticmethod
def _load_serialized_parts(phys_reader, pkg_srels, content_types):

    sparts = []
    part_walker = PackageReader._walk_phys_parts(phys_reader, pkg_srels)
    for (partname, blob, reltype, srels) in part_walker:
        content_type = content_types[partname]
        spart = _SerializedPart(partname, content_type, reltype, blob, srels)
        sparts.append(spart)
    return tuple(sparts)
