migrating into a subfield clobbers a previous whole field migration

MigrateExecutable::import calls checkRequirements() on the migration and stops the import if it throws an exception. One option would be to add something else in that method but just have it post a warning.

However, maybe a separate service that takes a yml file and runs it through a series of tests might be better. That would help debug issues with the file. Each test could be a plugin: a ClobberedSubfield plugin, one that checks if the source file exists, is readable, and has real data, etc.

We don't need new machinery for this.

MigrationPluginManager::processDefinition() is the place to check that the definition of a migration is well-formed.