Robo command to fetch git-submodules

[amitaibu]

I've continued to experiment with DDEV multi-sites and git submodules. 
My use case is that each subsite has its own repo, and the "base repo" should be able to plug & play different subsites.

Getting git-submoduels right can be hard and error prone (but IMO makes sense to use in this specific scenario). so I've added a Robo file that knows how to:

1. Cleanup previous submodules.
2. Add the new ones, based on a CSV file that has the subsite name, git repo, and git branch
3. Add symlinks for new subsites, for their config, so config split will work.
4. Copy an adapted settings.php to each subsite1. Re-create config.local.yaml so ddev will install those new sites
5. Run ddev restart

Complimenting this ./vendor/bin/robo fetch "./robo/sites-collection1.csv" command there's a ./vendor/bin/robo reset", to cleanup the directory and git.