Allow customizing the display of project file names when reading

Allow customizing the display of project file names when reading

To hopefully resolve a long-running discussion
(https://lists.gnu.org/archive/html/emacs-devel/2019-05/msg00162.html).

* lisp/progmodes/project.el (project-read-file-name-function):
New variable.
(project--read-file-absolute, project--read-file-cpd-relative):
New functions, possible values for the above.
(project-find-file-in): Use the introduced variable.
(project--completing-read-strict): Retain just the logic that fits
the name.