Refactor MigrationDrushCommandProgress::initializeProgress() to reflect use accurate count totals depending on operation

Problem/Motivation

Refactor MigrationDrushCommandProgress::initializeProgress() logic to use accurate counts depending on the process

• migrate:import: use $source->count() - $migration->getIdMap()->importedCount()
• migrate:rollback: use use processed count

When using the migrate:import drush command without additional options, the progress indicator uses the source total count when the the count could reflect the unprocessed items.

For example, a migration with a total count of 1000 items with with 500 items already processed. Executing `drush mim my_migration` displays a progress indicator representing the total count along with a percentage of the total count, such that
250/1000 [=======>--------------------] 25%

Where as the progress indicator would more accurately reflect the status by using the unprocessed item count,
250/500 [============>---------------] 50%

Steps to reproduce

Proposed resolution

TBD

Remaining tasks

• Is this accurate?
• Is this feature worth supporting?
• What are the options to implement?
• Which option is the most desirable?
• What are the steps to implement?

User interface changes

Yes, TBD

API changes

TBD

Data model changes

TBD