#Migrate critical

I do not have time now to make the agreed changes, but this commit should fix some of the failing tests.

Four of the migrations in the block_content module

• Use the embedded_data source plugin;
• Have the "Drupal 6" or "Drupal 7" migration tag (in fact, the migrations have both tags);
• Include source_module in the source configuration.

Maybe I should remove the source_module from those migrations, but for now I am changing the logic so that we do not get a deprecation message for these. See Comments #50, #56.

I think Comment #73 and the third option in Comment #69 are the same thing. Let's do that. (As soon as I have a little spare time.)

For the record: another option is to move all the source plugins (except those in the migrate module) to the migrate_drupal module.