Add support for recursive directory search in drush config:import --partial --source=/some/dir

[davearch]

Is your feature request related to a problem? Please describe.
I have a pretty big migration that I am working on and to make it more organized it would be nice to have subdirectories in my module with the migration config. For example:
./my_migration_module/
 |__ /config/
  |__ /install/
    |__ /distinct_migration_scenario/
      |__ migration_file.yml
      |__ migration_file2.yml
    |__ /distinct_migration_scenario2/
      |__ migration_file3.yml
    |__ migration_file4.yml
Currently if I try to run drush cim --partial --source=./my_migration_module/config/install
Only migration_file4 would be imported. But it would be nice to have all of them imported at once.

Describe the solution you'd like
I would like to define a single directory and have all the config files in it picked up recursively for import.

Describe alternatives you've considered
I could keep all the files in a single directory, but for organization it's becoming unwieldy. I could also use separate directories and just run the command with different directories each time. I could even put those all in a single drush command so it doesn't have to be redundant. So this is just a 'nice to have' feature.

Additional context
I am willing to work on this.