If a layer uses addpylib to add functions that are required to parse recipes, layerindex fails.

Recent example:

ERROR: Unable to read /opt/workdir/https___github_com_webOS-ports_meta-webos-ports/meta-luneos/recipes-core/images/luneos-emulator-appliance.bb: Traceback (most recent call last):
  File "/opt/workdir/git___git_openembedded_org_bitbake/lib/bb/data_smart.py", line 465, in expandWithRefs
    s = __expand_python_regexp__.sub(varparse.python_sub, s)
  File "/opt/workdir/git___git_openembedded_org_bitbake/lib/bb/data_smart.py", line 154, in python_sub
    value = utils.better_eval(codeobj, DataContext(self.d), {'d' : self.d})
  File "/opt/workdir/git___git_openembedded_org_bitbake/lib/bb/utils.py", line 489, in better_eval
    return eval(source, ctx, locals)
  File "Var <WEBOS_DISTRO_TOPDIR_BRANCH>", line 1, in <module>
NameError: name 'oebackport' is not defined

Many more of the same error in the following:
https://layers.openembedded.org/layerindex/layerupdate/9489081/
https://layers.openembedded.org/layerindex/layerupdate/9489082/

Where layerindex is missing 'oebackport' definition:
addpylib ${LAYERDIR}/lib oebackport

https://github.com/webOS-ports/meta-webos-ports/blob/1783bea818eeeda8a2e171529f46a00b14702e6a/meta-luneos/conf/layer.conf#L25