bug#63648: 29.0.90; project.el: with switch-use-entire-map, switch-project errors on non-project commands

[Juri Linkov <juri@linkov.net>]

>>>>> (Tangential further thought: I wonder if we could make C-x p work
>>>>> as a prefix for all commands automatically, so for example
>>>>> C-x p C-x C-j would open the project root,

I just noticed that 'C-x p C-x C-j' would open the parent directory
of the project root, not the root itself, since 'C-x C-j' in the root
jumps to its parent.

> I have an idea for how to make it work, which might be generally useful:
>
> Maybe commands/keybindings could support marking themselves as "prefix"
> commands, such that when you run describe-key and enter the prefix key
> sequence, describe-key accepts another key sequence after you finish the
> prefix key sequence, and interprets it with the altered keymaps that
> would be created by the prefix.  And then the help buffer would print
> the help for both commands.
>
> So then (describe-key (kbd "C-x p M-&")) would print help for both the
> new project-prefix command and async-shell-command.
>
> And also, (describe-key (kbd "C-x p c")) would print help for both
> project-prefix and project-compile, the latter being bound only by the
> transient created by C-x p.

Before doing this for 'C-x p' and 'C-x 4' it's possible already to test
this on the existing 'C-x p p' since currently

  (describe-key (kbd "C-x p p c"))

fails to describe 'project-prefix-map'.  And only prints help for
'project-switch-project' with

  (describe-key (kbd "C-x p p"))