YAML discovery does not take theme inheritance into account

Problem/Motivation

This problem occurs for plugin types with YAML discovery.

In Core, there is layout.

In contrib, at least the following modules have the problem in their plugin types:

• UI Styles
• UI Skins
• Layout Options
• UI Examples
• UI Patterns 1 made a workaround in https://git.drupalcode.org/project/ui_patterns/-/blob/8.x-1.x/modules/ui...

Steps to reproduce

Have two themes, one is a parent, the other is a child.

Ensure the machine name of the child is alphabetically before the machine name of the parent.

Declare a layout in the parent theme.
Override the layout in the child theme.

Result: the plugin is not overridden.

Proposed resolution

Remaining tasks

Found a solution.