'Return a |PackageReader| instance loaded with contents of *pkg_file*.'

@staticmethod
def from_file(pkg_file):

    phys_reader = PhysPkgReader(pkg_file)
    content_types = _ContentTypeMap.from_xml(phys_reader.content_types_xml)
    pkg_srels = PackageReader._srels_for(phys_reader, PACKAGE_URI)
    sparts = PackageReader._load_serialized_parts(phys_reader, pkg_srels, content_types)
    phys_reader.close()
    return PackageReader(content_types, pkg_srels, sparts)
