Add attribute discovery support for our plugin types

Opened an MR with the code from 🐛 Fix failing pipelines Active .

This merge request looks great and I think we can merge this. Since the discovery of normal annotations is not yet disabled, this shouldn't impact any plugins out in the wild.

Thanks for the feedback, good to hear!
I think I’ll still leave it open for a few days more before merging, just to be on the safe side.

I also now stumbled over #3099471: Deprecate "hidden" definition key in favor of "no_ui" → , which I had forgotten but makes sense – all other of our plugin types now have a $no_ui property, but processors only have $hidden, which we now had to make explicit. (Up to now we had silently accepted both.)
The good thing about this move is that people will have to manually (or semi-manually) switch from annotations to attributes anyways, so now would probably be a perfect moment to switch to $no_ui, if we want. Or to add both but deprecate $hidden right away.
Not sure whether the additional consistency is worth the hassle, though. Any opinions on this?

I think it's a good idea to do, but if we can create a rector rule, to automate the refactoring, it doesn't have to happen at the same time.

You’re probably right, no need to do both at the same time. Thanks!
Merged.