Deprecate migrate source plugins

Updated from discussion on the Migrate video call today, benjifisher, heddn, mikelutz and myself were present. 📌 [meeting] Migrate Meeting 2025-04-24 2100Z Active

Alternative proposal in !12617 which deprecates DrupalSqlBase but then skips the deprecation notices for core extensions of that class; any other extensions in contrib will trigger the message as normal.

Added some test coverage by removing test modules from the skip, and then expecting the deprecation in tests that use them. Let's see if anything else will fail now I copied in the other two source plugin deprecations from the original MR.

Added similar dynamic migration messages for the EmptySource plugin and I18nQueryTrait trait and skipped the core implementations of them, they should still trigger in contrib/custom code.

I like that, it’s pretty slick and much fewer lines than I was expecting to have to deal with. I’m inclined to try to get this merged in early in the 10.3 cycle so that contrib has time to deal with the fallout. It leaves the ‘hard’ problem of identifying each of the source plugins the legacy tests for the actual removal when the 12.x branch opens, but at least we don’t have to do that twice, once for @legacy annotations, and again for removal. And when we get around to doing it, we could just remove the suppressions to get a list of all affected tests easily enough.

mikelutz → changed the visibility of the branch 3507572-deprecate-migrate-source to hidden.

@longwave, Do we need to add the @deprecated annotation for DrupalSqlBase, or should that just be covered with the deprecation of the whole module eventually?

We can, because that will notify people via PHPStan as well as via tests or at runtime - but I suspect the PHPStan baseline will grow quite a bit.