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

def from_file(pkg_file):
        """
        Return a |PackageReader| instance loaded with contents of *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)