Add a sub-module for migrating from Acquia Search

The module is now working well and has test coverage. We also broadened its scope, it can now be used to migrate from any other Solr hosting provider, not just from Acquia Search.

Apart from the failing CI pipelines, there are two remaining problems that are unfortunately caused by bugs in other modules (unless I’m mistaken):

• Saving a copied index will make the Migration module “forget” that it was copied, displaying wrong information on the Migration Overview page and making switching of the index’s search views impossible. This is a bug in the Search API Solr module, see 🐛 Saving an index removes other modules’ third-party settings Active .
• When uninstall the Migration module (which generally makes sense to do after the migration is finished) all views touched by it are deleted. I think this is a bug in the Views module in Core – I reported it at 🐛 Uninstalling a module deletes all views that have third-party settings by that module Active .

Since it will hardly be possible to wait until the two bugs are fixed and we depend on the versions that contain those fixes, I now instead refactored the code to use a key-value store instead of third-party settings to store the migration history. This also makes for simpler code, so it’s a win-win.
Now just trying to get the pipelines green …

After several problems just getting the tests to run, and with consistent results, in Drupal 8, pipelines are now passing.

Merged this. I’d consider this experimental for now, I’ll create a new release once the feature has been tested thoroughly.

Automatically closed - issue fixed for 2 weeks with no activity.