Relative imports are intentionally avoided in UI code (`scripts/startup/bl_ui`) because it makes it impossible to run individual files in isolation.
Common workflow for making mock-ups and quick UI edits is to load the UI file in Blender's text editor with "Edit Source" operator, make changes, and run the script to see the changes. But in the current version it's impossible because files include relative imports, which don't allow Python to run scripts individually.
Pull Request: https://projects.blender.org/blender/blender/pulls/138246
5dd53fdb29