Migrations are configuration entities and are therefore built on top of the configuration system. The configuration system does not allow dots in data keys though (see also https://www.drupal.org/node/2297311 for more information on this problem), so migrations needed to change to not use dots in their data keys either.
process:
entity_type: constants.entity_type
bundle: type
field_name:
plugin: migration
migration: d6_taxonomy_vocabulary
source: vid
'settings.allowed_values.0.vocabulary': @field_name
'settings.allowed_values.0.parent': constants.parent
Dots in data keys appear in the process structure in reference to target keys in the migrated data. These dots were changed to a / notation. Dots also appear in the values of configuration element in migrations where referring to source constants. While this does not cause any issues in configuration, the separator for these elements were also changed for consistency to a / notation. Therefore the following structure became:
process:
entity_type: 'constants/entity_type'
bundle: type
field_name:
plugin: migration
migration: d6_taxonomy_vocabulary
source: vid
'settings/allowed_values/0/vocabulary': @field_name
'settings/allowed_values/0/parent': 'constants/parent'
