ProductVariation::prepareRow() is complex and brittle and perhaps doing things in the wrong place

Problem/Motivation

During development a few bugs have come from \Drupal\bigcommerce\Plugin\migrate\source\ProductVariation::prepareRow(). This code seems brittle and perhaps we're doing things wrong.

Proposed resolution

• Move all the processing that's done here into a new process plugin. So the source plugin doesn't require other migrations to have already been run.
• Try and make sure that all assumptions are catered for and clear exceptions are thrown when expectations not met.

Remaining tasks

Discuss

User interface changes

None

API changes

Possible new process plugin

Data model changes

None

Release notes snippet

N/a