bug#41572: 28.0.50; [PATCH] Support plain project marked with file .emacs-project

[Nikolay Kudryavtsev]

It's mostly to somewhat empower people to add new backends.

But being to chose backend order is helpful too, since lets say I'm a 
major mode developer. I add my project type to the end of the list. Then 
someone says "hey, my VC structure is non-standard so the project gets 
detected wrongly" and I can just point the user to reorder his list.

I don't think we should worry about the user experience getting worse 
too much, since anyone touching project-find-functions should know what 
they're doing.