Problem/Motivation
hook_widget_info_alter() is not called anymore.
Proposed resolution
Add AlterDecorator to the discovery object used by WidgetPluginManager.
Remaining tasks
Review the patch.
User interface changes
None.
API changes
None, really. hook_field_widget_info() wasn't called for a while, but that is really a regression, instead of this being an API change. Note that this is not marked critical or even major, even though it is a regression. That is because this patch is trivial enough that I don't fear we will ship without it. Feel free to elevate the priority if you disagree.