Remove getSourceModule() from MigrateSourceInterface

Problem/Motivation

Other than a couple of tests in the migrate_drupal module, getSourceModule() is only called in DrupalSqlBase::checkRequirements(). All of this logic should live there as a protected method that isn't on any interface.

Proposed resolution

Deprecate and remove MigrateSourceInterface::getSourceModule().

Remaining tasks

User interface changes

None

Introduced terminology

None

API changes

This issue removes a public method from MigrateSourceInterface.

Data model changes

None

Release notes snippet