Move and rename xref-find-regexp to the project package

Move and rename xref-find-regexp to the project package

* lisp/progmodes/project.el (project-find-regexp)
(project--read-regexp)
(project--find-regexp-in): New functions.

* lisp/progmodes/xref.el (xref--find-xrefs): Extract from
xref--show-xrefs.  Use in existing callers in place of that
function.
(xref--show-xrefs): Only do the "show" part.
(xref-find-regexp): Rename, more or less, to
project-or-libraries-find-regexp.