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

[Juri Linkov]

>> Then maybe the backend could be named 'project-file'
>> since a special file defines the project root.
>
> That's a little more meaningful, though too close to
> 'project-files'. 'project-markered' or 'project-markerfile' would probably
> be less ambiguous.

In 'project-filemarker' I misread "filemarker" as "filmmaker" :-)
Another possible name would be "fileroot".

> Suppose somebody puts it before 'vc' to use if for a purpose we did not
> design it for: make sure that some subproject 'foo' in their monorepo is
> considered a separate project. 'foo/Makefile' exists, so they add
> "Makefile" to project-fallback-markers, and it kind of seems to work.

There are two contradictory needs:

1. When a marker list contains both ".dir-locals.el" and "Makefile",
   it should ignore Makefile files in vc-based project subdirs, e.g.
   emacs/lisp/Makefile, etc.

2. OTOH, I often type 'C-x p g' to search all gems of the same
   ruby version in e.g. ~/.rbenv/versions/2.7.4/lib/ruby/gems
   But it finds ~/.rbenv/.git and tries to search all ruby versions.
   I could manually add .dir-locals.el only to a particular version's
   subdir.  But how to override ~/.rbenv/.git?  Maybe by changing
   the order of backends in project-find-functions?
   Then the fallback won't be the last backend anymore.
   Also the backend priorities will be changed globally
   for all other projects, and 'C-x p g' in emacs/lisp
   will find emacs/lisp/Makefile to override emacs/.git.