Add uniquify-get-unique-names (bug#77312)

Add uniquify-get-unique-names (bug#77312)

This new function provides an interface to uniquify.el which doesn't
change the actual names of the buffers.  This is useful for any commands
which deal with a subset of all buffers; for example, project.el.

* lisp/uniquify.el (uniquify-rationalize--generic): Add.
(uniquify-rationalize, uniquify-rationalize-a-list)
(uniquify-rationalize-conflicting-sublist): Explicitly pass
RENAME-BUFFER-FN and GET-BUFFER-FN.
(uniquify--stateless-curname, uniquify-get-unique-names): Add.

(cherry picked from commit 79cd1cc30e0c9a5f058279dc6f618e5dc22a1945)