Restore configure-only option

Here we go... A suggested workflow for using this to do custom Drupal-to-Drupal migrations:

1. Install a fresh empty D8 site, enabling all modules for which you wish to migrate data.
2. Run the drush migrate-upgrade command with the --configure-only option. This generates migration configuration entities in the D8 database (config table).
3. Create a custom module containing only a .info.yml file (with dependencies on migrate_plus and migrate_drupal) and a config/install directory.
4. Export your site configuration, e.g. drush cex --destination=/tmp/export
5. Copy the migration configuration that was generated by migrate-upgrade - be sure *not* to copy the default group configuration, which is defined by migrate_plus:
   cp /tmp/export/migrate_plus.migration.* /tmp/export/migrate_plus.migration_group.migrate_*.yml migrate_custom/config/install/
6. Look at that migrate_plus.migration_group.* file - you'll see your database configuration captured there. In most cases, what you'll want to do is define your database connection in settings.php with those credentials under the key that is configured there - you won't want to commit the credentials to your git repo.
7. Edit the generated .yml files to reflect your custom migration path.
8. Reinstall D8, enable your custom module and migrate_tools, and proceed to work with your Drupal migrations as you would with any custom migration. Hint: you'll probably want config_devel so you can edit .yml files in config/install and immediately test your changes.

Committed, thanks!