Reproduces steps:

I have the following directory

```

\---myproject
    |  .project
    |
    +---subproject1
    |   \---.git
    |   |   Makefile
    |
    \---subproject2
        \---.git
        |   Makefile

```

1. Open Emacs -Q

2. M-x eval-expression `(setq debug-on-error t)`

3. M-x eval-expression `(setq project-vc-extra-root-markers '(".project"))`

4. Open the .project file in Emacs

5. M-x eval-expression `(project-current)`

    Result: `(vc nil "e:/work/myproject/")` OK

6. M-x eval-expression `(project-files (project-current))`

    Result: Debugger entered--Lisp error: (error "File listing failed:
FIND: Parameter format not co...")
  error("File listing failed: %s" "FIND: Parameter format not correct\n")

The root cause is that `project--files-in-directory` in project.el is using
`find-program` ("find") which does not exist on Windows.

Is it possible to use more portable code? For example,
`directory-files-recursively`? Otherwise NTEmacs users will have to install
CYGWIN or MSYS2.

P.s. I have used Projectile before, which allows you to place a .projectile
file in `myproject` and everything was OK on Windows. Also if I focus on a
git folder e.g. `subproject1` in the example above, the `project-files`
also works well.

Emacs 29.3 on Windows 10.

Thanks.

Best Regards,
Siyuan Chen