Fix usage of cons cells in grep-find-ignored-files

Fix usage of cons cells in grep-find-ignored-files

grep-find-ignored-files is documented to also include cons
cells, not just globs, but there were two places outside grep.el
where we were using it as if it was only a string list.

To fix this, add a helper function named grep-find-ignored-files
which handles grep-find-ignored-files properly and returns the
list of globs, and use it everywhere.

* lisp/progmodes/grep.el (grep--filter-list-by-dir)
(grep-find-ignored-files): New functions.
(rgrep-find-ignored-directories): Use grep--filter-list-by-dir.
(lgrep, rgrep-default-command):
Use grep-find-ignored-files function.
* lisp/dired-aux.el (dired-do-find-regexp):
Use grep-find-ignored-files function.
* lisp/progmodes/project.el (project-ignores):
Use grep-find-ignored-files function, if bound. (bug#71115)