The export of add-on translations to PO files suffered from two
issues:
1. To find the best file candidate for each language, the exporter
compares ISO codes. One of the codes it tried to compare with is
the invalid `__POT__` (`settings.PARSER_TEMPLATE_ID`).
This would raise an assert in
`bl_i18n_utils.utils.locale_explode()` because its implementation
is different from the original `bpy_translations.locale_explode()`
when given invalid values.
This key is now filtered out to avoid the exception.
2. With "Update Existing" enabled, it tried to update the nonexistant
`trans.msgs` of the translations. The proper dictionary is
`trans.trans`.
