Bring back ECA's own condition plugin manager
Problem/Motivation
The intention to use the existing condition plugin manager from Drupal core had been to use, what's already there. As we always try, we don't want to re-invent the wheel. However, in this case it seems to have been a bad move. This is because core's conditions have numerous limitations and unresolved issues. This can be seen in many code comments that highlight such issues and indicate, as well, that there should be a lot of work to come to not only fix them but also re-factor that whole API. In reality, those condition plugins are only used in the block visibility context, and they only properly work there.
Trying to add more conditions that then also work for ECA is causing more difficulties than the benefit of avoiding redundancy could ever deliver. The rules module has been there before too, see e.g. #2811519: Blocks do not appear after being placed with the Rules module enabled (or other missing schemata for Condition plugins).
Proposed resolution
De-couple ECA's condition plugins and bring back ECA's own condition plugin manager.