Move source_module from Migrate to Migrate Drupal

Problem/Motivation

Discussed at the migrate meeting.

The source_module and destination_module are needed for Drupal and should not be in Migrate, they probably should be in Migrate Drupal. Let's see what needs to be done to get them out of Migrate.

I have a feeling that this is a duplicate even though I couldn't find a similar issue.

Proposed resolution

Keep getSourceModule in migrate but only allow it to check for source_module in configuration. This allows source_module to be set in migration yaml so that the source plugin can be used in migrate_drupal.

Add getSourceModule in migrate_drupal to search for the source_module in the definition.

Adjust tests accordingly.

Remaining tasks

1. Review
2. Create a follow up for destination_module - #3438187: Move destination_module from Migrate to Migrate Drupal
3. This issue is postponed on #3421014: Convert MigrateSource plugin discovery to attributes.

User interface changes

API changes

Data model changes

Release notes snippet