bug#43153: Add new command project-find-dir (and move binding of project-dired)

[Juri Linkov]

>>> Could we move `project-dired' to `C-x p D' in Emacs 28.1, while awaiting
>>> this new keybinding `C-x p d'?  Moving it later will be much harder.
>>
>> If current `project-dired' actually means "project-root-dired",
>> then it makes sense to bind it to `C-x p D' like `C-x v D' (vc-root-diff).
>
> Does it makes sense to rename it, as well?
>
> For the sake of completion I mention that the same command is named
> `projectile-dired' in projectile, where it is bound here to `C-c p D'.
> I believe you can use whatever prefix you want, though.

Maybe then better to rename `project-dired' to `project-root-dired',
then bind `C-x p D' to `project-root-dired' without the prompt,
and bind `project-dired' to `C-x p d' with a prompt that
asks for a directory like `C-x p f' (project-find-file).