Fix 'C-u C-x p g' globally and 'A' in dired-mode

Fix 'C-u C-x p g' globally and 'A' in dired-mode

* lisp/progmodes/project.el (project-find-regexp): Ensure the
DEFAULT-DIRECTORY is set correctly for the 'C-u' case (bug#70888).

* lisp/progmodes/project.el (project--files-in-directory):
Ensure that the DIR argument ends with a slash --
'dired-do-find-regexp' passes it differently, for example.

* lisp/progmodes/xref.el (xref--group-name-for-display): Ensure
the project-relative and absolute display modes work well for
groups with "relative" file names.
(xref--analyze, xref--add-log-current-defun): Change accordingly.

* test/lisp/progmodes/project-tests.el
(project-find-regexp-with-prefix): New test.