Rebase project-find-regexp on top of project-files

Rebase project-find-regexp on top of project-files

* lisp/progmodes/project.el (project--files-in-directory):
New function.
(project-files, project-find-regexp): Use it.
(project--dir-ignores): New function.
(project--find-regexp-in): Remove.
(project--process-file-region): New function.
(project--find-regexp-in-files): New function.
(project-find-regexp, project-or-external-find-regexp): Use it, and
project-files as well.