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

[Juri Linkov]

>> And indeed with the following patch replacing the current definition of
>> 'project-switch-project' with just:
>>
>>   (defun project-switch-project (dir)
>>     (interactive (list (funcall project-prompter)))
>>     (setq next-default-directory dir))
>
> This would be a new "prefix command", like the `universal-argument`,
> `C-x RET c`, the `ofw-other-window`, vc-edit-next-command,
> `ofw-other-frame` from the `other-frame-window` package, and a few more.
>
> In order for this new prefix to interact correctly with those other
> ones, it should probably use `prefix-command-preserve-state` and it
> would be good to make it show some visual feedback via
> `prefix-command-echo-keystrokes-functions`.

Here is the same function turned into a prefix command,
so it supports both cases:

1. 'C-x p p d'      -- keys from project-prefix-map
2. 'C-x p p C-x d'  -- global keybindings

```
(defun project-switch-project (dir)
  (interactive (list (funcall project-prompter)))
  (if (symbolp project-switch-commands)
      (let ((default-directory dir))
        (call-interactively project-switch-commands))
    (let* ((echofun (lambda () "[switch-project]"))
           (postfun (lambda () (remove-hook
                                'prefix-command-echo-keystrokes-functions
                                echofun))))
      (setq next-default-directory dir)
      (add-hook 'prefix-command-echo-keystrokes-functions echofun)
      (message (project--keymap-prompt))
      (prefix-command-update)
      (set-transient-map project-prefix-map nil postfun))))
```

If this is conceptually ok, then more customization could be ported
from project--switch-project-command such as project-switch-use-entire-map.