bug#54359: 29.0.50; [PATCH] Undo breaking change to project interface

[Eli Zaretskii]

> From: dick <dick.r.chiang@gmail.com>
> Date: Sat, 12 Mar 2022 16:47:53 -0500
> 
> >From 793d1311b28701931a9343e5196adce82da7833d Mon Sep 17 00:00:00 2001
> From: dickmao <dick.r.chiang@gmail.com>
> Date: Sat, 12 Mar 2022 16:39:28 -0500
> Subject: [PATCH] Avoid breaking the world's `project-current` calls
> 
> mitigate 86969f9.
> 
> * lisp/progmodes/project.el (cl-generic, cl-lib):
> Rectify the requires
> (project-try-vc): Cache the backend outside project "struct."
> (project-root): Cache the backend outside project "struct."
> (project-files): Cache the backend outside project "struct."
> (project-ignores): Cache the backend outside project "struct."
> * test/lisp/progmodes/project-tests.el
> (project-sparing-backend-discovery): Test it.

Please provide the rationale for this changeset.  In particular, what
did the offending commit break, how to reproduce the breakage, and why
the best solution is to revert that commit?