cephfs-shell: move source to separate subdirectory

cephfs-shell: move source to separate subdirectory

This ensures the package discovery done by python setuptools >= 61
doesn't get confused when building cephfs-shell and cephfs-top.
This commit moves cephfs-shell to a separate "shell" subdirectory,
which is the same approach we've already got with the cephfs-top
tool being in a separate "top" subdirectory.

Fixes: https://tracker.ceph.com/issues/56658
Signed-off-by: Tim Serong <tserong@suse.com>
(cherry picked from commit dc69033763cc116c6ccdf1f97149a74248691042)